#ctci

MyQueue.java

import java.util.Stack;

public class MyQueue<E>{
    private Stack<E> s1;
    private Stack<E> s2;
    private int N = 0;
    
    public MyQueue(){
        s1 = new Stack<E>();
        s2 = new Stack<E>();
    }
    
    public void enqueue(E val){
        s1.push(val);
        N++;
    }
    
    public E dequeue() throws Exception{
        if(N == 0) throw new Exception("The queue is already empty!");
        
        if(s2.size() == 0) moveFromS1toS2();
        
        N--;
        
        return s2.pop();
    }
    
    public int size(){
        return N;
    }
    
    private void moveFromS1toS2(){
        while(s1.size() > 0){
            E tmp = s1.pop();
            s2.push(tmp);
        }
    }
    
    public static void main(String[] args) throws Exception {
		// TODO Auto-generated method stub
		MyQueue<Integer> queue = new MyQueue<Integer>();
		queue.enqueue(1);
		queue.enqueue(2);
		queue.enqueue(3);
		queue.enqueue(4);

		System.out.println(queue.dequeue());
		System.out.println(queue.dequeue());
		System.out.println(queue.dequeue());
		System.out.println(queue.dequeue());
	}
}