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Reverse stack using in memory stack #2653

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Merged
siriak merged 2 commits into from
Oct 23, 2021
Merged

Reverse stack using in memory stack #2653

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Reverse stack
ishika22 Oct 16, 2021
6b3fa7f
Merge branch 'master' into master
siriak Oct 23, 2021
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70 changes: 70 additions & 0 deletions DataStructures/Stacks/ReverseStack.java
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package DataStructures.Stacks;

import java.util.Scanner;
import java.util.Stack;

/**
* Reversal of a stack using recursion.
*
* @author Ishika Agarwal, 2021
*/

public class ReverseStack {

public static void main(String args[]){

Scanner sc = new Scanner(System.in);
System.out.println("Enter the number of elements you wish to insert in the stack");
int n = sc.nextInt();
int i;
Stack<Integer> stack = new Stack<Integer>();
System.out.println("Enter the stack elements");
for(i = 0; i < n ; i++)
stack.push(sc.nextInt());
sc.close();
reverseStack(stack);
System.out.println("The reversed stack is:");
while(!stack.isEmpty()){
System.out.print(stack.peek()+",");
stack.pop();
}

}

private static void reverseStack(Stack<Integer> stack) {
if(stack.isEmpty()){
return;
}

//Store the topmost element
int element = stack.peek();
//Remove the topmost element
stack.pop();

//Reverse the stack for the leftover elements
reverseStack(stack);

//Insert the topmost element to the bottom of the stack
insertAtBottom(stack,element);

}

private static void insertAtBottom(Stack<Integer> stack, int element) {

if(stack.isEmpty()){
//When stack is empty, insert the element so it will be present at the bottom of the stack
stack.push(element);
return;
}

int ele = stack.peek();
/*Keep popping elements till stack becomes empty. Push the elements once the topmost element has
moved to the bottom of the stack.
*/
stack.pop();
insertAtBottom(stack, element);

stack.push(ele);
}

}