Implement Stack using Queues Java

PROGRAM TO IMPLEMENT STACK USING QUEUES

import java.util.*;

  

class GfG {

  

    static class Stack {

        // Two inbuilt queues

        static Queue<Integer> q1 = new LinkedList<Integer>();

        static Queue<Integer> q2 = new LinkedList<Integer>();

  

        // To maintain current number of

        // elements

        static int curr_size;

  

        Stack()

        {

            curr_size = 0;

        }

  

        static void push(int x)

        {

            curr_size++;

  

            // Push x first in empty q2

            q2.add(x);

  

            // Push all the remaining

            // elements in q1 to q2.

            while (!q1.isEmpty()) {

                q2.add(q1.peek());

                q1.remove();

            }

  

            // swap the names of two queues

            Queue<Integer> q = q1;

            q1 = q2;

            q2 = q;

        }

  

        static void pop()

        {

  

            // if no elements are there in q1

            if (q1.isEmpty())

                return;

            q1.remove();

            curr_size--;

        }

  

        static int top()

        {

            if (q1.isEmpty())

                return -1;

            return q1.peek();

        }

  

        static int size()

        {

            return curr_size;

        }

    }

  

    // driver code

    public static void main(String[] args)

    {

        Stack s = new Stack();

        s.push(1);

        s.push(2);

        s.push(3);

  

        System.out.println("current size: " + s.size());

        System.out.println(s.top());

        s.pop();

        System.out.println(s.top());

        s.pop();

        System.out.println(s.top());

  

        System.out.println("current size: " + s.size());

    }

}




OUTPUT:

current size: 3
3
2
1
current size: 1

CREDITS: https://www.geeksforgeeks.org/implement-stack-using-queue/