stackQueue.java

import java.util.*;
import java.util.LinkedList;
//IMPLEMENT A STACK USING TWO QUEUES
public class stackQueue {
    static class Stack{
        static Queue<Integer> q1 = new LinkedList<>();
        static Queue<Integer> q2 = new LinkedList<>();
        public static boolean isEmpty(){
            return q1.isEmpty() && q2.isEmpty();
        }
        //PUSH O(n)
        public static void push(int data){
            if(!q1.isEmpty()){
                q1.add(data);
            }else{
                q2.add(data);
            }
        }
        //POP O(1)
        public static int pop(){
            if(isEmpty()){
                System.out.println("Empty stack");
                return -1;
            }
            int top = -1;
            //case 1 - elements are in queue1
            if(!q1.isEmpty()){
                while(!q1.isEmpty()){
                    top = q1.remove();
                    if(q1.isEmpty()){
                        break;
                    }
                    q2.add(top);
                }
            }else{ //case 2 -elements in queue2
                while(!q2.isEmpty()){
                    top = q2.remove();
                    if(q2.isEmpty()){
                        break;
                    }
                    q1.add(top);
                }
                } 
            return top;  
        }
         //PEEK O(1)
        public static int peek(){
            if(isEmpty()){
                System.out.println("Empty stack");
                return -1;
            }
            int top = -1;
            //case 1 - elements are in queue1
            if(!q1.isEmpty()){
                while(!q1.isEmpty()){
                    top = q1.remove();
                    q2.add(top);
                }
            }else{ //case 2 -elements in queue2
                while(!q2.isEmpty()){
                    top = q2.remove();
                    q1.add(top);
                }
                } 
            return top;  
        }
    }
    public static void main(String[] args) {
        Stack s = new Stack();
        s.push(1);
        s.push(2);
        s.push(3);
        while(!s.isEmpty()){
            System.out.println(s.peek());
            s.pop();
        }
    }
    
}