The idea of stack simulating queue

The characteristic of the stack : Last in, first out

The characteristics of the queue are : fifo

Stack simulation queue , It requires elements to be first in, first out , But the characteristic of stack is last in first out , This requires two stacks to simulate queues ( Accord with the characteristic of first in first out )

1. First create two stacks ( Stack 1:in, Stack 2:out)

2. According to the characteristics of queue first in first out : Stack 1 Into the , Stack 2 Out

When you join the team , According to the characteristics of stack LIFO : Just ask for stack 2 You can only join the team when it is empty ; Otherwise, it will stack 2 Elements in stack 1,in.push(out.pop()), Until the stack 2 It's empty ( All on the stack 1 in ) To join the team in.push()

Empathy , When the team , Request stack 1 It's empty time , Before leaving the team ; Otherwise, it will stack 1 The elements in Push 2 out.push(in.pop()), Until the stack 1 It's empty ( All on the stack 2 Only in the middle can you leave the team ),out.pop( )

  Code implementation ：

package com.my.stack;
import java.util.Stack;
public class Test {
	public static void main(String[] args) {
		MyQueue<String> queue = new MyQueue<String>();
		queue.offer("A1");
		queue.offer("A2");
		queue.offer("A3");
		queue.offer("A4");
		System.out.println(queue.poll());
		//  Traverse the queue 
		while(!queue.isEmpty()) {
			System.out.println(queue.poll());
		}
	}
}
//  Stack simulation queue 
class MyQueue<E>{
	private Stack<E> in = new Stack<E>(); //  Join the team 
	private Stack<E> out = new Stack<E>(); //  Out of the stack 
	//  Determines if the queue is empty 
	public boolean isEmpty() {
		return in.size()==0 && out.size()==0;
	}
	//  The team 
	public void offer(E e) {
		while(!out.isEmpty()) {
			in.push(out.pop());
		}
		in.push(e);		
	}
	//  Out of the team 
	public E poll() {
		while(!in.isEmpty()) {
			out.push(in.pop());
		}
		return out.pop();	
	}
}