diseña una stack tal que getMinimum () debería ser O (1)

Esta es una de las preguntas de una entrevista. Necesita diseñar una stack que contenga un valor entero tal que la función getMinimum () devuelva el elemento mínimo en la stack.

Por ejemplo: considere el siguiente ejemplo

 caso 1

 5 -> TOP
 1
 4
 6
 2

 Cuando se llama a getMinimum () debe devolver 1, que es el elemento mínimo 
 en la stack. 

 caso # 2

 stack.pop ()
 stack.pop ()

 Nota: tanto 5 como 1 están fuera de la stack.  Entonces después de esto, la stack
 parece,

 4 -> TOP
 6
 2

 Cuando se llama a getMinimum () debe devolver 2, que es el mínimo en el 
 astackr.

Constriants:

  1. getMinimum debe devolver el valor mínimo en O (1)
  2. La restricción de espacio también debe tenerse en cuenta al diseñarla y, si usa espacio adicional, debe tener espacio constante.

EDITAR: Esto falla la restricción de “espacio constante” – básicamente duplica el espacio requerido. Dudo mucho que haya una solución que no lo haga, sin destruir la complejidad del tiempo de ejecución en algún lugar (por ejemplo, hacer push / pop O (n)). Tenga en cuenta que esto no cambia la complejidad del espacio requerido, por ejemplo, si tiene una stack con requisitos de espacio O (n), esto todavía será O (n) solo con un factor constante diferente.

Solución de espacio no constante

Mantenga una stack “duplicada” de “mínimo de todos los valores más bajos en la stack”. Cuando saltas la stack principal, abre la stack mínima también. Cuando empuja la stack principal, presione el elemento nuevo o el valor mínimo actual, el que sea más bajo. getMinimum() se implementa como minStack.peek() .

Entonces, usando su ejemplo, tendríamos:

 Real stack Min stack 5 --> TOP 1 1 1 4 2 6 2 2 2 

Después de aparecer dos veces, obtienes:

 Real stack Min stack 4 2 6 2 2 2 

Por favor, avíseme si esto no es suficiente información. Es simple cuando lo asimila, pero puede tomar un poco de rasguño en la cabeza al principio 🙂

(La desventaja, por supuesto, es que duplica el requisito de espacio. Sin embargo, el tiempo de ejecución no sufre significativamente, es decir, sigue siendo la misma complejidad).

EDITAR: Hay una variación que es un poco más complicada, pero tiene mejor espacio en general. Todavía tenemos la stack mínima, pero solo salimos de ella cuando el valor que sacamos de la stack principal es igual al de la stack mínima. Solo presionamos a la stack mínima cuando el valor que se empuja en la stack principal es menor o igual que el valor mínimo actual. Esto permite duplicar valores mínimos. getMinimum() sigue siendo solo una operación de vistazo. Por ejemplo, tomando la versión original y presionando 1 nuevamente, obtendríamos:

 Real stack Min stack 1 --> TOP 1 5 1 1 2 4 6 2 

Apareciendo desde los puntos arriba de las dos stacks porque 1 == 1, dejando:

 Real stack Min stack 5 --> TOP 1 1 2 4 6 2 

Aparecer nuevamente solo aparece de la stack principal, porque 5> 1:

 Real stack Min stack 1 1 4 2 6 2 

Estallar de nuevo aparece en ambas stacks porque 1 == 1:

 Real stack Min stack 4 2 6 2 

Esto termina con la misma complejidad en el peor espacio posible (el doble de la stack original) pero mucho mejor uso de espacio si raramente obtenemos un “nuevo mínimo o igual”.

EDITAR: Aquí hay una implementación del esquema malvado de Pete. No lo he probado a fondo, pero creo que está bien 🙂

 using System.Collections.Generic; public class FastMinStack { private readonly Stack stack = new Stack(); // Could pass this in to the constructor private readonly IComparer comparer = Comparer.Default; private T currentMin; public T Minimum { get { return currentMin; } } public void Push(T element) { if (stack.Count == 0 || comparer.Compare(element, currentMin) <= 0) { stack.Push(currentMin); stack.Push(element); currentMin = element; } else { stack.Push(element); } } public T Pop() { T ret = stack.Pop(); if (comparer.Compare(ret, currentMin) == 0) { currentMin = stack.Pop(); } return ret; } } 

Agregue un campo para mantener el valor mínimo y actualícelo durante Pop () y Push (). De esa forma getMinimum () será O (1), pero Pop () y Push () tendrán que trabajar un poco más.

Si aparece un valor mínimo, Pop () será O (n); de lo contrario, ambos seguirán siendo O (1). Al cambiar el tamaño de Push () se convierte en O (n) según la implementación de Stack.

Aquí hay una implementación rápida

 public sealed class MinStack { private int MinimumValue; private readonly Stack Stack = new Stack(); public int GetMinimum() { if (IsEmpty) { throw new InvalidOperationException("Stack is empty"); } return MinimumValue; } public int Pop() { var value = Stack.Pop(); if (value == MinimumValue) { MinimumValue = Stack.Min(); } return value; } public void Push(int value) { if (IsEmpty || value < MinimumValue) { MinimumValue = value; } Stack.Push(value); } private bool IsEmpty { get { return Stack.Count() == 0; } } } 
 public class StackWithMin { int min; int size; int[] data = new int[1024]; public void push ( int val ) { if ( size == 0 ) { data[size] = val; min = val; } else if ( val < min) { data[size] = 2 * val - min; min = val; assert (data[size] < min); } else { data[size] = val; } ++size; // check size and grow array } public int getMin () { return min; } public int pop () { --size; int val = data[size]; if ( ( size > 0 ) && ( val < min ) ) { int prevMin = min; min += min - val; return prevMin; } else { return val; } } public boolean isEmpty () { return size == 0; } public static void main (String...args) { StackWithMin stack = new StackWithMin(); for ( String arg: args ) stack.push( Integer.parseInt( arg ) ); while ( ! stack.isEmpty() ) { int min = stack.getMin(); int val = stack.pop(); System.out.println( val + " " + min ); } System.out.println(); } } 

Almacena el mínimo actual explícitamente, y si el mínimo cambia, en lugar de presionar el valor, empuja un valor con la misma diferencia al otro lado del nuevo mínimo (si min = 7 y empuja 5, empuja 3 en su lugar (5- | 7-5 | = 3) y establece min a 5; si luego muestra 3 cuando min es 5, ve que el valor reventado es menor que min, entonces invierte el procedimiento para obtener 7 para el nuevo min, luego devuelve el valor anterior min). Como cualquier valor que no causa un cambio, el mínimo actual es mayor que el mínimo actual, tiene algo que se puede usar para diferenciar entre los valores que cambian el mínimo y los que no.

En los idiomas que usan enteros de tamaño fijo, está tomando prestado un poco de espacio de la representación de los valores, por lo que puede subdesbordar y la afirmación fallará. Pero, de lo contrario, es un espacio adicional constante y todas las operaciones siguen siendo O (1).

Las stacks que están basadas en cambio en listas vinculadas tienen otros lugares de los que puede tomar prestado un poco, por ejemplo, en C el bit menos significativo del siguiente puntero, o en Java el tipo de los objetos en la lista vinculada. Para Java, esto significa que se usa más espacio en comparación con una stack contigua, ya que tiene la sobrecarga del objeto por enlace:

 public class LinkedStackWithMin { private static class Link { final int value; final Link next; Link ( int value, Link next ) { this.value = value; this.next = next; } int pop ( LinkedStackWithMin stack ) { stack.top = next; return value; } } private static class MinLink extends Link { MinLink ( int value, Link next ) { super( value, next ); } int pop ( LinkedStackWithMin stack ) { stack.top = next; int prevMin = stack.min; stack.min = value; return prevMin; } } Link top; int min; public LinkedStackWithMin () { } public void push ( int val ) { if ( ( top == null ) || ( val < min ) ) { top = new MinLink(min, top); min = val; } else { top = new Link(val, top); } } public int pop () { return top.pop(this); } public int getMin () { return min; } public boolean isEmpty () { return top == null; } 

En C, la sobrecarga no está allí, y puede tomar prestado el lsb del siguiente puntero:

 typedef struct _stack_link stack_with_min; typedef struct _stack_link stack_link; struct _stack_link { size_t next; int value; }; stack_link* get_next ( stack_link* link ) { return ( stack_link * )( link -> next & ~ ( size_t ) 1 ); } bool is_min ( stack_link* link ) { return ( link -> next & 1 ) ! = 0; } void push ( stack_with_min* stack, int value ) { stack_link *link = malloc ( sizeof( stack_link ) ); link -> next = ( size_t ) stack -> next; if ( (stack -> next == 0) || ( value == stack -> value ) ) { link -> value = stack -> value; link -> next |= 1; // mark as min } else { link -> value = value; } stack -> next = link; } etc.; 

Sin embargo, ninguno de estos son verdaderamente O (1). No requieren más espacio en la práctica, ya que explotan agujeros en las representaciones de números, objetos o punteros en estos idiomas. Pero una máquina teórica que utilizara una representación más compacta requeriría agregar un bit adicional a esa representación en cada caso.

Encontré una solución que satisface todas las restricciones mencionadas (operaciones de tiempo constante) y espacio extra constante .

La idea es almacenar la diferencia entre el valor mínimo y el número de entrada, y actualizar el valor mínimo si ya no es el mínimo.

El código es el siguiente:

 public class MinStack { long min; Stack stack; public MinStack(){ stack = new Stack<>(); } public void push(int x) { if (stack.isEmpty()) { stack.push(0L); min = x; } else { stack.push(x - min); //Could be negative if min value needs to change if (x < min) min = x; } } public int pop() { if (stack.isEmpty()) return; long pop = stack.pop(); if (pop < 0) { long ret = min min = min - pop; //If negative, increase the min value return (int)ret; } return (int)(pop + min); } public int top() { long top = stack.peek(); if (top < 0) { return (int)min; } else { return (int)(top + min); } } public int getMin() { return (int)min; } } 

El crédito va a: https://leetcode.com/discuss/15679/share-my-java-solution-with-only-one-stack

Bueno, ¿cuáles son las limitaciones de tiempo de ejecución de push y pop ? Si no se requiere que sean constantes, simplemente calcule el valor mínimo en esas dos operaciones (convirtiéndolas en O ( n )). De lo contrario, no veo cómo se puede hacer esto con un espacio adicional constante.

Aquí está mi Código que se ejecuta con O (1). El código anterior que publiqué tenía un problema cuando aparece el elemento mínimo. Modifiqué mi código Esta usa otra stack que mantiene el elemento mínimo presente en la stack sobre el elemento empujado actual.

  class StackDemo { int[] stk = new int[100]; int top; public StackDemo() { top = -1; } public void Push(int value) { if (top == 100) Console.WriteLine("Stack Overflow"); else stk[++top] = value; } public bool IsEmpty() { if (top == -1) return true; else return false; } public int Pop() { if (IsEmpty()) { Console.WriteLine("Stack Underflow"); return 0; } else return stk[top--]; } public void Display() { for (int i = top; i >= 0; i--) Console.WriteLine(stk[i]); } } class MinStack : StackDemo { int top; int[] stack = new int[100]; StackDemo s1; int min; public MinStack() { top = -1; s1 = new StackDemo(); } public void PushElement(int value) { s1.Push(value); if (top == 100) Console.WriteLine("Stack Overflow"); if (top == -1) { stack[++top] = value; stack[++top] = value; } else { // stack[++top]=value; int ele = PopElement(); stack[++top] = ele; int a = MininmumElement(min, value); stack[++top] = min; stack[++top] = value; stack[++top] = a; } } public int PopElement() { if (top == -1) return 1000; else { min = stack[top--]; return stack[top--]; } } public int PopfromStack() { if (top == -1) return 1000; else { s1.Pop(); return PopElement(); } } public int MininmumElement(int a,int b) { if (a > b) return b; else return a; } public int StackTop() { return stack[top]; } public void DisplayMinStack() { for (int i = top; i >= 0; i--) Console.WriteLine(stack[i]); } } class Program { static void Main(string[] args) { MinStack ms = new MinStack(); ms.PushElement(15); ms.PushElement(2); ms.PushElement(1); ms.PushElement(13); ms.PushElement(5); ms.PushElement(21); Console.WriteLine("Min Stack"); ms.DisplayMinStack(); Console.WriteLine("Minimum Element:"+ms.StackTop()); ms.PopfromStack(); ms.PopfromStack(); ms.PopfromStack(); ms.PopfromStack(); Console.WriteLine("Min Stack"); ms.DisplayMinStack(); Console.WriteLine("Minimum Element:" + ms.StackTop()); Thread.Sleep(1000000); } } 

Usé un tipo diferente de stack. Aquí está la implementación.

 // // main.cpp // Eighth // // Created by chaitanya on 4/11/13. // Copyright (c) 2013 cbilgika. All rights reserved. // #include  #include  using namespace std; struct stack { int num; int minnum; }a[100]; void push(int n,int m,int &top) { top++; if (top>=100) { cout<<"Stack Full"; cout<::min(),num; cout<<"Enter the list to push (-1 to stop): "; cin>>num; while (num!=-1) { if (top == -1) { min = num; push(num, min, top); } else{ if (num < min) { min = num; } push(num, min, top); } cin>>num; } print(top); get_min(top); return 0; } 

Salida:

 Enter the list to push (-1 to stop): 5 1 4 6 2 -1 Stack: (5,5) (1,1) (4,1) (6,1) (2,1) Minimum element is: 1 

Intentalo. Creo que responde la pregunta. El segundo elemento de cada par proporciona el valor mínimo visto cuando se insertó ese elemento.

Estoy publicando el código completo aquí para encontrar el mínimo y el máximo en una stack determinada.

La complejidad del tiempo será O (1) ..

 package com.java.util.collection.advance.datastructure; /** * * @author vsinha * */ public abstract interface Stack { /** * Placing a data item on the top of the stack is called pushing it * @param element * */ public abstract void push(E element); /** * Removing it from the top of the stack is called popping it * @return the top element */ public abstract E pop(); /** * Get it top element from the stack and it * but the item is not removed from the stack, which remains unchanged * @return the top element */ public abstract E peek(); /** * Get the current size of the stack. * @return */ public abstract int size(); /** * Check whether stack is empty of not. * @return true if stack is empty, false if stack is not empty */ public abstract boolean empty(); } package com.java.util.collection.advance.datastructure; @SuppressWarnings("hiding") public abstract interface MinMaxStack extends Stack { public abstract int min(); public abstract int max(); } package com.java.util.collection.advance.datastructure; import java.util.Arrays; /** * * @author vsinha * * @param  */ public class MyStack implements Stack { private E[] elements =null; private int size = 0; private int top = -1; private final static int DEFAULT_INTIAL_CAPACITY = 10; public MyStack(){ // If you don't specify the size of stack. By default, Stack size will be 10 this(DEFAULT_INTIAL_CAPACITY); } @SuppressWarnings("unchecked") public MyStack(int intialCapacity){ if(intialCapacity <=0){ throw new IllegalArgumentException("initial capacity can't be negative or zero"); } // Can't create generic type array elements =(E[]) new Object[intialCapacity]; } @Override public void push(E element) { ensureCapacity(); elements[++top] = element; ++size; } @Override public E pop() { E element = null; if(!empty()) { element=elements[top]; // Nullify the reference elements[top] =null; --top; --size; } return element; } @Override public E peek() { E element = null; if(!empty()) { element=elements[top]; } return element; } @Override public int size() { return size; } @Override public boolean empty() { return size == 0; } /** * Increases the capacity of this Stack by double of its current length instance, * if stack is full */ private void ensureCapacity() { if(size != elements.length) { // Don't do anything. Stack has space. } else{ elements = Arrays.copyOf(elements, size *2); } } @Override public String toString() { return "MyStack [elements=" + Arrays.toString(elements) + ", size=" + size + ", top=" + top + "]"; } } package com.java.util.collection.advance.datastructure; /** * Time complexity will be O(1) to find min and max in a given stack. * @author vsinha * */ public class MinMaxStackFinder extends MyStack implements MinMaxStack { private MyStack minStack; private MyStack maxStack; public MinMaxStackFinder (int intialCapacity){ super(intialCapacity); minStack =new MyStack(); maxStack =new MyStack(); } public void push(Integer element) { // Current element is lesser or equal than min() value, Push the current element in min stack also. if(!minStack.empty()) { if(min() >= element) { minStack.push(element); } } else{ minStack.push(element); } // Current element is greater or equal than max() value, Push the current element in max stack also. if(!maxStack.empty()) { if(max() <= element) { maxStack.push(element); } } else{ maxStack.push(element); } super.push(element); } public Integer pop(){ Integer curr = super.pop(); if(curr !=null) { if(min() == curr) { minStack.pop(); } if(max() == curr){ maxStack.pop(); } } return curr; } @Override public int min() { return minStack.peek(); } @Override public int max() { return maxStack.peek(); } @Override public String toString() { return super.toString()+"\nMinMaxStackFinder [minStack=" + minStack + "\n maxStack=" + maxStack + "]" ; } } // You can use the below program to execute it. package com.java.util.collection.advance.datastructure; import java.util.Random; public class MinMaxStackFinderApp { public static void main(String[] args) { MinMaxStack stack =new MinMaxStackFinder(10); Random random =new Random(); for(int i =0; i< 10; i++){ stack.push(random.nextInt(100)); } System.out.println(stack); System.out.println("MAX :"+stack.max()); System.out.println("MIN :"+stack.min()); stack.pop(); stack.pop(); stack.pop(); stack.pop(); stack.pop(); System.out.println(stack); System.out.println("MAX :"+stack.max()); System.out.println("MIN :"+stack.min()); } } 

Avísame si enfrentas algún problema

Gracias, Vikash

Puede ampliar su clase de stack original y simplemente agregarle el seguimiento mínimo. Deje que la clase de padres original maneje todo lo demás como de costumbre.

 public class StackWithMin extends Stack { private Stack min; public StackWithMin() { min = new Stack<>(); } public void push(int num) { if (super.isEmpty()) { min.push(num); } else if (num <= min.peek()) { min.push(num); } super.push(num); } public int min() { return min.peek(); } public Integer pop() { if (super.peek() == min.peek()) { min.pop(); } return super.pop(); } } 

Aquí está mi solución en Java usando la lista de Me gusta.

 class Stack{ int min; Node top; static class Node{ private int data; private Node next; private int min; Node(int data, int min){ this.data = data; this.min = min; this.next = null; } } void push(int data){ Node temp; if(top == null){ temp = new Node(data,data); top = temp; top.min = data; } if(top.min > data){ temp = new Node(data,data); temp.next = top; top = temp; } else { temp = new Node(data, top.min); temp.next = top; top = temp; } } void pop(){ if(top != null){ top = top.next; } } int min(){ return top.min; } 

}

Aquí está mi versión de implementación.

  struct MyStack {
     elemento int;
     int * CurrentMiniAddress;
  };

  void Push (valor int)
  {
     // Crea tu estructura y puebla el valor
     MyStack S = nuevo MyStack ();
     S-> elemento = valor;

     if (Stack.Empty ())
     {    
         // Como la stack está vacía, apunta a CurrentMiniAddress a sí mismo
         S-> CurrentMiniAddress = S;

     }
     más
     {
          // La stack no está vacía

          // Recuperar el elemento superior.  No Pop ()
          MyStack * TopElement = Stack.Top ();

          // Recuerde siempre que el elemento TOP apunta a la
          // elemento mínimo en la stack entera
          if (S-> elemento CurrentMiniAddress-> elemento)
          {
             // Si el valor actual es el mínimo en toda la stack
             // entonces S apunta a sí mismo
             S-> CurrentMiniAddress = S;
          }
              más
              {
                  // Entonces este no es el mínimo en toda la stack
                  // No te preocupes, TOP tiene el elemento mínimo
                  S-> CurrentMiniAddress = TopElement-> CurrentMiniAddress;
              }

     }
         Stack.Add (S);
  }

  void Pop ()
  {
      if (! Stack.Empty ())
      {
         Stack.Pop ();
      }  
  }

  int GetMinimum (Stack & stack)
  {
        if (! stack.Empty ())
        {
             MyStack * Top = stack.top ();
             // Top siempre apunta al minimumx
             return Top-> CurrentMiniAddress-> element;
         }
  }
 #include struct stack { int data; int mindata; }a[100]; void push(int *tos,int input) { if (*tos > 100) { printf("overflow"); return; } (*tos)++; a[(*tos)].data=input; if (0 == *tos) a[*tos].mindata=input; else if (a[*tos -1].mindata < input) a[*tos].mindata=a[*tos -1].mindata; else a[*tos].mindata=input; } int pop(int * tos) { if (*tos <= -1) { printf("underflow"); return -1; } return(a[(*tos)--].data); } void display(int tos) { while (tos > -1) { printf("%d:%d\t",a[tos].data,a[tos].mindata); tos--; } } int min(int tos) { return(a[tos].mindata); } int main() { int tos=-1,x,choice; while(1) { printf("press 1-push,2-pop,3-mindata,4-display,5-exit "); scanf("%d",&choice); switch(choice) { case 1: printf("enter data to push"); scanf("%d",&x); push(&tos,x); break; case 2: printf("the poped out data=%d ",pop(&tos)); break; case 3: printf("The min peeped data:%d",min(tos)); break; case 4: printf("The elements of stack \n"); display(tos); break; default: exit(0); } } 

Encontré esta solución aquí

 struct StackGetMin { void push(int x) { elements.push(x); if (minStack.empty() || x <= minStack.top()) minStack.push(x); } bool pop() { if (elements.empty()) return false; if (elements.top() == minStack.top()) minStack.pop(); elements.pop(); return true; } bool getMin(int &min) { if (minStack.empty()) { return false; } else { min = minStack.top(); return true; } } stack elements; stack minStack; }; 
 struct Node { let data: Int init(_ d:Int){ data = d } } struct Stack { private var backingStore = [Node]() private var minArray = [Int]() mutating func push(n:Node) { backingStore.append(n) minArray.append(n.data) minArray.sort(>) minArray } mutating func pop() -> Node? { if(backingStore.isEmpty){ return nil } let n = backingStore.removeLast() var found = false minArray = minArray.filter{ if (!found && $0 == n.data) { found = true return false } return true } return n } func min() -> Int? { return minArray.last } } 
  class MyStackImplementation{ private final int capacity = 4; int min; int arr[] = new int[capacity]; int top = -1; public void push ( int val ) { top++; if(top <= capacity-1){ if(top == 0){ min = val; arr[top] = val; } else if(val < min){ arr[top] = arr[top]+min; min = arr[top]-min; arr[top] = arr[top]-min; } else { arr[top] = val; } System.out.println("element is pushed"); } else System.out.println("stack is full"); } public void pop () { top--; if(top > -1){ min = arr[top]; } else {min=0; System.out.println("stack is under flow");} } public int min(){ return min; } public boolean isEmpty () { return top == 0; } public static void main(String...s){ MyStackImplementation msi = new MyStackImplementation(); msi.push(1); msi.push(4); msi.push(2); msi.push(10); System.out.println(msi.min); msi.pop(); msi.pop(); msi.pop(); msi.pop(); msi.pop(); System.out.println(msi.min); } } 
 class FastStack { private static class StackNode { private Integer data; private StackNode nextMin; public StackNode(Integer data) { this.data = data; } public Integer getData() { return data; } public void setData(Integer data) { this.data = data; } public StackNode getNextMin() { return nextMin; } public void setNextMin(StackNode nextMin) { this.nextMin = nextMin; } } private LinkedList stack = new LinkedList<>(); private StackNode currentMin = null; public void push(Integer item) { StackNode node = new StackNode(item); if (currentMin == null) { currentMin = node; node.setNextMin(null); } else if (item < currentMin.getData()) { StackNode oldMinNode = currentMin; node.setNextMin(oldMinNode); currentMin = node; } stack.addFirst(node); } public Integer pop() { if (stack.isEmpty()) { throw new EmptyStackException(); } StackNode node = stack.peek(); if (currentMin == node) { currentMin = node.getNextMin(); } stack.removeFirst(); return node.getData(); } public Integer getMinimum() { if (stack.isEmpty()) { throw new NoSuchElementException("Stack is empty"); } return currentMin.getData(); } } 

Aquí está mi Código que se ejecuta con O (1). Aquí utilicé el par vectorial que contiene el valor que empujó y también contiene el valor mínimo hasta este valor empujado.

Aquí está mi versión de la implementación de C ++.

 vector >A; int sz=0; // to keep track of the size of vector class MinStack { public: MinStack() { A.clear(); sz=0; } void push(int x) { int mn=(sz==0)?x: min(A[sz-1].second,x); //find the minimum value upto this pushed value A.push_back(make_pair(x,mn)); sz++; // increment the size } void pop() { if(sz==0) return; A.pop_back(); // pop the last inserted element sz--; // decrement size } int top() { if(sz==0) return -1; // if stack empty return -1 return A[sz-1].first; // return the top element } int getMin() { if(sz==0) return -1; return A[sz-1].second; // return the minimum value at sz-1 } }; 
  **The task can be acheived by creating two stacks:** import java.util.Stack; /* * * Find min in stack using O(n) Space Complexity */ public class DeleteMinFromStack { void createStack(Stack primary, Stack minStack, int[] arr) { /* Create main Stack and in parallel create the stack which contains the minimum seen so far while creating main Stack */ primary.push(arr[0]); minStack.push(arr[0]); for (int i = 1; i < arr.length; i++) { primary.push(arr[i]); if (arr[i] <= minStack.peek())// Condition to check to push the value in minimum stack only when this urrent value is less than value seen at top of this stack */ minStack.push(arr[i]); } } int findMin(Stack secStack) { return secStack.peek(); } public static void main(String args[]) { Stack primaryStack = new Stack(); Stack minStack = new Stack(); DeleteMinFromStack deleteMinFromStack = new DeleteMinFromStack(); int[] arr = { 5, 5, 6, 8, 13, 1, 11, 6, 12 }; deleteMinFromStack.createStack(primaryStack, minStack, arr); int mimElement = deleteMinFromStack.findMin(primaryStack, minStack); /** This check for algorithm when the main Stack Shrinks by size say i as in loop below */ for (int i = 0; i < 2; i++) { primaryStack.pop(); } System.out.println(" Minimum element is " + mimElement); } } /* here in have tried to add for loop wherin the main tack can be shrinked/expaned so we can check the algorithm */ 

Una implementación práctica para encontrar el mínimo en una stack de objetos diseñados por el usuario, llamada: Escuela

The Stack va a almacenar las escuelas en Stack según el rango asignado a una escuela en una región específica, digamos que findMin () le otorga a la escuela el número máximo de solicitudes de Admisiones, que a su vez debe ser definido por el comparador que utiliza el rango asociado con las escuelas en la temporada anterior.

 The Code for same is below: package com.practical; import java.util.Collections; import java.util.Iterator; import java.util.LinkedList; import java.util.List; import java.util.Stack; public class CognitaStack { public School findMin(Stack stack, Stack minStack) { if (!stack.empty() && !minStack.isEmpty()) return (School) minStack.peek(); return null; } public School removeSchool(Stack stack, Stack minStack) { if (stack.isEmpty()) return null; School temp = stack.peek(); if (temp != null) { // if(temp.compare(stack.peek(), minStack.peek())<0){ stack.pop(); minStack.pop(); // } // stack.pop(); } return stack.peek(); } public static void main(String args[]) { Stack stack = new Stack(); Stack minStack = new Stack(); List lst = new LinkedList(); School s1 = new School("Polam School", "London", 3); School s2 = new School("AKELEY WOOD SENIOR SCHOOL", "BUCKINGHAM", 4); School s3 = new School("QUINTON HOUSE SCHOOL", "NORTHAMPTON", 2); School s4 = new School("OAKLEIGH HOUSE SCHOOL", " SWANSEA", 5); School s5 = new School("OAKLEIGH-OAK HIGH SCHOOL", "Devon", 1); School s6 = new School("BritishInter2", "Devon", 7); lst.add(s1); lst.add(s2); lst.add(s3); lst.add(s4); lst.add(s5); lst.add(s6); Iterator itr = lst.iterator(); while (itr.hasNext()) { School temp = itr.next(); if ((minStack.isEmpty()) || (temp.compare(temp, minStack.peek()) < 0)) { // minStack.peek().equals(temp) stack.push(temp); minStack.push(temp); } else { minStack.push(minStack.peek()); stack.push(temp); } } CognitaStack cogStack = new CognitaStack(); System.out.println(" Minimum in Stack is " + cogStack.findMin(stack, minStack).name); cogStack.removeSchool(stack, minStack); cogStack.removeSchool(stack, minStack); System.out.println(" Minimum in Stack is " + ((cogStack.findMin(stack, minStack) != null) ? cogStack.findMin(stack, minStack).name : "Empty")); } } 

También el objeto escolar es el siguiente:

 package com.practical; import java.util.Comparator; public class School implements Comparator { String name; String location; int rank; public School(String name, String location, int rank) { super(); this.name = name; this.location = location; this.rank = rank; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((location == null) ? 0 : location.hashCode()); result = prime * result + ((name == null) ? 0 : name.hashCode()); result = prime * result + rank; return result; } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; School other = (School) obj; if (location == null) { if (other.location != null) return false; } else if (!location.equals(other.location)) return false; if (name == null) { if (other.name != null) return false; } else if (!name.equals(other.name)) return false; if (rank != other.rank) return false; return true; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getLocation() { return location; } public void setLocation(String location) { this.location = location; } public int getRank() { return rank; } public void setRank(int rank) { this.rank = rank; } public int compare(School o1, School o2) { // TODO Auto-generated method stub return o1.rank - o2.rank; } } class SchoolComparator implements Comparator { public int compare(School o1, School o2) { return o1.rank - o2.rank; } } 

This Example covers the following: 1. Implementation of Stack for User defined Objects, here, School 2. Implementation for the hashcode() and equals() method using all fields of Objects to be compared 3. A practical implementation for the scenario where we rqeuire to get the Stack contains operation to be in order of O(1)

Here’s the PHP implementation of what explained in Jon Skeet’s answer as the slightly better space complexity implementation to get the maximum of stack in O(1).

 storage, $value) : false; } /** * Pops an element off the stack. * * @return int */ public function pop() { return array_pop($this->storage); } /** * See what's on top of the stack. * * @return int|bool */ public function top() { return empty($this->storage) ? false : end($this->storage); } // ------------------------------------------------------------------------ // Magic methods // ------------------------------------------------------------------------ /** * String representation of the stack. * * @return string */ public function __toString() { return implode('|', $this->storage); } } // End of BaseIntegerStack class /** * The stack implementation with getMax() method in O(1). */ class Stack extends BaseIntegerStack { /** * Internal stack to keep track of main stack max values. * * @var BaseIntegerStack */ private $maxStack; /** * Stack class constructor. * * Dependencies are injected. * * @param BaseIntegerStack $stack Internal stack. * * @return void */ public function __construct(BaseIntegerStack $stack) { $this->maxStack = $stack; } // ------------------------------------------------------------------------ // Public API // ------------------------------------------------------------------------ /** * Prepends an item into the stack maintaining max values. * * @param int $value New item to push to the stack. * * @return bool */ public function push($value) { if ($this->isNewMax($value)) { $this->maxStack->push($value); } parent::push($value); } /** * Pops an element off the stack maintaining max values. * * @return int */ public function pop() { $popped = parent::pop(); if ($popped == $this->maxStack->top()) { $this->maxStack->pop(); } return $popped; } /** * Finds the maximum of stack in O(1). * * @return int * @see push() */ public function getMax() { return $this->maxStack->top(); } // ------------------------------------------------------------------------ // Internal helpers // ------------------------------------------------------------------------ /** * Checks that passing value is a new stack max or not. * * @param int $new New integer to check. * * @return boolean */ private function isNewMax($new) { return empty($this->maxStack) OR $new > $this->maxStack->top(); } } // End of Stack class // ------------------------------------------------------------------------ // Stack Consumption and Test // ------------------------------------------------------------------------ $stack = new Stack( new BaseIntegerStack ); $stack->push(9); $stack->push(4); $stack->push(237); $stack->push(5); $stack->push(556); $stack->push(15); print "Stack: $stack\n"; print "Max: {$stack->getMax()}\n\n"; print "Pop: {$stack->pop()}\n"; print "Pop: {$stack->pop()}\n\n"; print "Stack: $stack\n"; print "Max: {$stack->getMax()}\n\n"; print "Pop: {$stack->pop()}\n"; print "Pop: {$stack->pop()}\n\n"; print "Stack: $stack\n"; print "Max: {$stack->getMax()}\n"; // Here's the sample output: // // Stack: 9|4|237|5|556|15 // Max: 556 // // Pop: 15 // Pop: 556 // // Stack: 9|4|237|5 // Max: 237 // // Pop: 5 // Pop: 237 // // Stack: 9|4 // Max: 9 

Here is the C++ implementation of Jon Skeets Answer . It might not be the most optimal way of implementing it, but it does exactly what it’s supposed to.

 class Stack { private: struct stack_node { int val; stack_node *next; }; stack_node *top; stack_node *min_top; public: Stack() { top = nullptr; min_top = nullptr; } void push(int num) { stack_node *new_node = nullptr; new_node = new stack_node; new_node->val = num; if (is_empty()) { top = new_node; new_node->next = nullptr; min_top = new_node; new_node->next = nullptr; } else { new_node->next = top; top = new_node; if (new_node->val <= min_top->val) { new_node->next = min_top; min_top = new_node; } } } void pop(int &num) { stack_node *tmp_node = nullptr; stack_node *min_tmp = nullptr; if (is_empty()) { std::cout << "It's empty\n"; } else { num = top->val; if (top->val == min_top->val) { min_tmp = min_top->next; delete min_top; min_top = min_tmp; } tmp_node = top->next; delete top; top = tmp_node; } } bool is_empty() const { return !top; } void get_min(int &item) { item = min_top->val; } }; 

And here is the driver for the class

 int main() { int pop, min_el; Stack my_stack; my_stack.push(4); my_stack.push(6); my_stack.push(88); my_stack.push(1); my_stack.push(234); my_stack.push(2); my_stack.get_min(min_el); cout << "Min: " << min_el << endl; my_stack.pop(pop); cout << "Popped stock element: " << pop << endl; my_stack.pop(pop); cout << "Popped stock element: " << pop << endl; my_stack.pop(pop); cout << "Popped stock element: " << pop << endl; my_stack.get_min(min_el); cout << "Min: " << min_el << endl; return 0; } 

Salida:

 Min: 1 Popped stock element: 2 Popped stock element: 234 Popped stock element: 1 Min: 4 

We can do this in O(n) time and O(1) space complexity, like so:

 class MinStackOptimized: def __init__(self): self.stack = [] self.min = None def push(self, x): if not self.stack: # stack is empty therefore directly add self.stack.append(x) self.min = x else: """ Directly add (x-self.min) to the stack. This also ensures anytime we have a negative number on the stack is when x was less than existing minimum recorded thus far. """ self.stack.append(x-self.min) if x < self.min: # Update x to new min self.min = x def pop(self): x = self.stack.pop() if x < 0: """ if popped element was negative therefore this was the minimum element, whose actual value is in self.min but stored value is what contributes to get the next min. (this is one of the trick we use to ensure we are able to get old minimum once current minimum gets popped proof is given below in pop method), value stored during push was: (x - self.old_min) and self.min = x therefore we need to backtrack these steps self.min(current) - stack_value(x) actually implies to x (self.min) - (x - self.old_min) which therefore gives old_min back and therefore can now be set back as current self.min. """ self.min = self.min - x def top(self): x = self.stack[-1] if x < 0: """ As discussed above anytime there is a negative value on stack, this is the min value so far and therefore actual value is in self.min, current stack value is just for getting the next min at the time this gets popped. """ return self.min else: """ if top element of the stack was positive then it's simple, it was not the minimum at the time of pushing it and therefore what we did was x(actual) - self.min(min element at current stage) let's say `y` therefore we just need to reverse the process to get the actual value. Therefore self.min + y, which would translate to self.min + x(actual) - self.min, thereby giving x(actual) back as desired. """ return x + self.min def getMin(self): # Always self.min variable holds the minimum so for so easy peezy. return self.min 

I think you can simply use a LinkedList in your stack implementation.

First time you push a value, you put this value as the linkedlist head.

then each time you push a value, if the new value < head.data, make a prepand operation ( which means the head becomes the new value )

if not, then make an append operation.

When you make a pop(), you check if min == linkedlist.head.data, if yes, then head=head.next;

Aquí está mi código.

 public class Stack { int[] elements; int top; Linkedlists min; public Stack(int n) { elements = new int[n]; top = 0; min = new Linkedlists(); } public void realloc(int n) { int[] tab = new int[n]; for (int i = 0; i < top; i++) { tab[i] = elements[i]; } elements = tab; } public void push(int x) { if (top == elements.length) { realloc(elements.length * 2); } if (top == 0) { min.pre(x); } else if (x < min.head.data) { min.pre(x); } else { min.app(x); } elements[top++] = x; } public int pop() { int x = elements[--top]; if (top == 0) { } if (this.getMin() == x) { min.head = min.head.next; } elements[top] = 0; if (4 * top < elements.length) { realloc((elements.length + 1) / 2); } return x; } public void display() { for (Object x : elements) { System.out.print(x + " "); } } public int getMin() { if (top == 0) { return 0; } return this.min.head.data; } public static void main(String[] args) { Stack stack = new Stack(4); stack.push(2); stack.push(3); stack.push(1); stack.push(4); stack.push(5); stack.pop(); stack.pop(); stack.pop(); stack.push(1); stack.pop(); stack.pop(); stack.pop(); stack.push(2); System.out.println(stack.getMin()); stack.display(); } } 
 public class MinStack{ private final LinkedList mainStack = new LinkedList(); private final LinkedList minStack = new LinkedList(); private final Comparator comparator; public MinStack(Comparator comparator) { this.comparator = comparator; } /** * Pushes an element onto the stack. * * * @param e the element to push */ public void push(E e) { mainStack.push(e); if(minStack.isEmpty()) { minStack.push(e); } else if(comparator.compare(e, minStack.peek())<=0) { minStack.push(e); } else { minStack.push(minStack.peek()); } } /** * Pops an element from the stack. * * * @throws NoSuchElementException if this stack is empty */ public E pop() { minStack.pop(); return mainStack.pop(); } /** * Returns but not remove smallest element from the stack. Return null if stack is empty. * */ public E getMinimum() { return minStack.peek(); } @Override public String toString() { StringBuilder sb = new StringBuilder(); sb.append("Main stack{"); for (E e : mainStack) { sb.append(e.toString()).append(","); } sb.append("}"); sb.append(" Min stack{"); for (E e : minStack) { sb.append(e.toString()).append(","); } sb.append("}"); sb.append(" Minimum = ").append(getMinimum()); return sb.toString(); } public static void main(String[] args) { MinStack st = new MinStack(Comparators.INTEGERS); st.push(2); Assert.assertTrue("2 is min in stack {2}", st.getMinimum().equals(2)); System.out.println(st); st.push(6); Assert.assertTrue("2 is min in stack {2,6}", st.getMinimum().equals(2)); System.out.println(st); st.push(4); Assert.assertTrue("2 is min in stack {2,6,4}", st.getMinimum().equals(2)); System.out.println(st); st.push(1); Assert.assertTrue("1 is min in stack {2,6,4,1}", st.getMinimum().equals(1)); System.out.println(st); st.push(5); Assert.assertTrue("1 is min in stack {2,6,4,1,5}", st.getMinimum().equals(1)); System.out.println(st); st.pop(); Assert.assertTrue("1 is min in stack {2,6,4,1}", st.getMinimum().equals(1)); System.out.println(st); st.pop(); Assert.assertTrue("2 is min in stack {2,6,4}", st.getMinimum().equals(2)); System.out.println(st); st.pop(); Assert.assertTrue("2 is min in stack {2,6}", st.getMinimum().equals(2)); System.out.println(st); st.pop(); Assert.assertTrue("2 is min in stack {2}", st.getMinimum().equals(2)); System.out.println(st); st.pop(); Assert.assertTrue("null is min in stack {}", st.getMinimum()==null); System.out.println(st); } } 
 using System; using System.Collections.Generic; using System.IO; using System.Linq; namespace Solution { public class MinStack { public MinStack() { MainStack=new Stack(); Min=new Stack(); } static Stack MainStack; static Stack Min; public void Push(int item) { MainStack.Push(item); if(Min.Count==0 || item 

Saw a brilliant solution here: https://www.geeksforgeeks.org/design-a-stack-that-supports-getmin-in-o1-time-and-o1-extra-space/

Bellow is the python code I wrote by following the algorithm:

 class Node: def __init__(self, value): self.value = value self.next = None class MinStack: def __init__(self): self.head = None self.min = float('inf') # @param x, an integer def push(self, x): if self.head == None: self.head = Node(x) self.min = x else: if x >= self.min: n = Node(x) n.next = self.head self.head = n else: v = 2 * x - self.min n = Node(v) n.next = self.head self.head = n self.min = x # @return nothing def pop(self): if self.head: if self.head.value < self.min: self.min = self.min * 2 - self.head.value self.head = self.head.next # @return an integer def top(self): if self.head: if self.head.value < self.min: self.min = self.min * 2 - self.head.value return self.min else: return self.head.value else: return -1 # @return an integer def getMin(self): if self.head: return self.min else: return -1 

To getMin elements from Stack. We have to use Two stack .ie Stack s1 and Stack s2.

  1. Initially, both stacks are empty, so add elements to both stacks

———————Recursively call Step 2 to 4———————–

  1. if New element added to stack s1.Then pop elements from stack s2

  2. compare new elments with s2. which one is smaller , push to s2.

  3. pop from stack s2(which contains min element)

Code looks like:

 package Stack; import java.util.Stack; public class getMin { Stack s1= new Stack(); Stack s2 = new Stack(); void push(int x) { if(s1.isEmpty() || s2.isEmpty()) { s1.push(x); s2.push(x); } else { s1. push(x); int y = (Integer) s2.pop(); s2.push(y); if(x < y) s2.push(x); } } public Integer pop() { int x; x=(Integer) s1.pop(); s2.pop(); return x; } public int getmin() { int x1; x1= (Integer)s2.pop(); s2.push(x1); return x1; } public static void main(String[] args) { getMin s = new getMin(); s.push(10); s.push(20); s.push(30); System.out.println(s.getmin()); s.push(1); System.out.println(s.getmin()); } } 

I think only push operation suffers, is enough. My implementation includes a stack of nodes. Each node contain the data item and also the minimum on that moment. This minimum is updated each time a push operation is done.

Here are some points for understanding:

  • I implemented the stack using Linked List.

  • A pointer top always points to the last pushed item. When there is no item in that stack top is NULL.

  • When an item is pushed a new node is allocated which has a next pointer that points to the previous stack and top is updated to point to this new node.

Only difference with normal stack implementation is that during push it updates a member min for the new node.

Please have a look at code which is implemented in C++ for demonstration purpose.

 /* * Implementation of Stack that can give minimum in O(1) time all the time * This solution uses same data structure for minimum variable, it could be implemented using pointers but that will be more space consuming */ #include  using namespace std; typedef struct stackLLNodeType stackLLNode; struct stackLLNodeType { int item; int min; stackLLNode *next; }; class DynamicStack { private: int stackSize; stackLLNode *top; public: DynamicStack(); ~DynamicStack(); void push(int x); int pop(); int getMin(); int size() { return stackSize; } }; void pushOperation(DynamicStack& p_stackObj, int item); void popOperation(DynamicStack& p_stackObj); int main () { DynamicStack stackObj; pushOperation(stackObj, 3); pushOperation(stackObj, 1); pushOperation(stackObj, 2); popOperation(stackObj); popOperation(stackObj); popOperation(stackObj); popOperation(stackObj); pushOperation(stackObj, 4); pushOperation(stackObj, 7); pushOperation(stackObj, 6); popOperation(stackObj); popOperation(stackObj); popOperation(stackObj); popOperation(stackObj); return 0; } DynamicStack::DynamicStack() { // initialization stackSize = 0; top = NULL; } DynamicStack::~DynamicStack() { stackLLNode* tmp; // chain memory deallocation to avoid memory leak while (top) { tmp = top; top = top->next; delete tmp; } } void DynamicStack::push(int x) { // allocate memory for new node assign to top if (top==NULL) { top = new stackLLNode; top->item = x; top->next = NULL; top->min = top->item; } else { // allocation of memory stackLLNode *tmp = new stackLLNode; // assign the new item tmp->item = x; tmp->next = top; // store the minimum so that it does not get lost after pop operation of later minimum if (x < top->min) tmp->min = x; else tmp->min = top->min; // update top to new node top = tmp; } stackSize++; } int DynamicStack::pop() { // check if stack is empty if (top == NULL) return -1; stackLLNode* tmp = top; int curItem = top->item; top = top->next; delete tmp; stackSize--; return curItem; } int DynamicStack::getMin() { if (top == NULL) return -1; return top->min; } void pushOperation(DynamicStack& p_stackObj, int item) { cout<<"Just pushed: "< 

And the output of the program looks like this:

 Just pushed: 3 Current stack min: 3 Current stack size: 1 Just pushed: 1 Current stack min: 1 Current stack size: 2 Just pushed: 2 Current stack min: 1 Current stack size: 3 Just popped: 2 Current stack min: 1 Current stack size: 2 Just popped: 1 Current stack min: 3 Current stack size: 1 Just popped: 3 No minimum. Stack is empty. Current stack size: 0 Cannot pop. Stack is empty. Just pushed: 4 Current stack min: 4 Current stack size: 1 Just pushed: 7 Current stack min: 4 Current stack size: 2 Just pushed: 6 Current stack min: 4 Current stack size: 3 Just popped: 6 Current stack min: 4 Current stack size: 2 Just popped: 7 Current stack min: 4 Current stack size: 1 Just popped: 4 No minimum. Stack is empty. Current stack size: 0 Cannot pop. Stack is empty. 
 public interface IMinStack> { public void push(T val); public T pop(); public T minValue(); public int size(); } 

 import java.util.Stack; public class MinStack> implements IMinStack { private Stack stack = new Stack(); private Stack minStack = new Stack(); @Override public void push(T val) { stack.push(val); if (minStack.isEmpty() || val.compareTo(minStack.peek()) < 0) minStack.push(val); } @Override public T pop() { T val = stack.pop(); if ((false == minStack.isEmpty()) && val.compareTo(minStack.peek()) == 0) minStack.pop(); return val; } @Override public T minValue() { return minStack.peek(); } @Override public int size() { return stack.size(); } } 
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