How to Sort a Stack using Merge Sort?

Interviewer may ask you this question if you have more than 3 years of experience in java development. So lets have a look, how to sort a stack using merge sort?

Lets see sample input and output for better understanding:

Algorithm
For this we will follow these below two steps,
1. Break the stack into two parts by using two temporary stack.

2. When only one element remains on a Stack, Merge it.

Lets have a look on the program,

package com.codebyakram;
 
import java.util.Stack;
 
public class SortStack {
 
    public static void main(String args[]) {
        Stack stack = new Stack();
        stack.push(5);
        stack.push(9);
        stack.push(4);
        stack.push(1);
        stack.push(2);
        stack.push(-1);

 
        sort(stack);
 
        System.out.println(stack);
    }
 
    private static void sort(Stack stack) {
        Stack s1 = new Stack();
        Stack s2 = new Stack();
 
        while (stack.size() != 0) {
            if (stack.size() % 2 == 0) {
                s1.push(stack.pop());
            } else {
                s2.push(stack.pop());
            }
        }
 
        if (s1.size() > 1) {
            sort(s1);
        }
 
        if (s2.size() > 1) {
            sort(s2);
        }
 
        merge(s1, s2, stack);
    }
 
    private static void merge(Stack s1, Stack s2, Stack stack) {
        Stack mergedStack = new Stack();
        while (!s1.isEmpty() && !s2.isEmpty()) {
            if ((Integer) s1.peek() < (Integer) s2.peek()) {
                mergedStack.push(s2.pop());
            } else {
                mergedStack.push(s1.pop());
            }
        }
 
        while (!s1.isEmpty()) {
            mergedStack.push(s1.pop());
        }
 
        while (!s2.isEmpty()) {
            mergedStack.push(s2.pop());
        }
 
        while (!mergedStack.isEmpty()) {
            stack.push(mergedStack.pop());
        }
    }
}