a33stackUsingQueue.java

import java.util.LinkedList;
import java.util.Queue;
public class a33stackUsingQueue {
    Queue<Integer> queue1 = new LinkedList<>();
    Queue<Integer> queue2 = new LinkedList<>();

    public void push(int val) {
        while (!queue1.isEmpty()){
            queue2.offer(queue1.poll());
        }
        queue1.offer(val);
        while (!queue2.isEmpty()){
            queue1.offer(queue2.poll());
        }
    }
    public int peek() {
        if(queue1.isEmpty()){
            System.out.println("Stack is empty");
            return -1;
        }
        return queue1.peek();
    }
    public int pop() {
        if(queue1.isEmpty()){
            System.out.println("Stack is empty");
            return -1;
        }
        return queue1.poll();
    }
    public static void main(String[] args) {

        a33stackUsingQueue stack = new a33stackUsingQueue();
        stack.push(10);
        System.out.println( stack.peek()); // 10
        stack.push(20);
        stack.push(30);
        stack.push(40);
        System.out.println(stack.peek()); // 40
        System.out.println(stack.pop()); // 40
        System.out.println(stack.peek()); // 30


    }
}