利用栈Stack实现队列(Queue)

实现说明：

入队时，将元素压入s1;

出队时，推断s2是否为空，如不为空，则直接弹出顶元素；如为空。则将s1的元素逐个“倒入”s2。把最后一个元素弹出并出队;

这个思路，避免了重复“倒”栈，仅在须要时才“倒”一次。

package com.knowledgeStudy.threadLocal;

import java.util.Stack;

public class MyQueue {

Stack<Object> s1 = new Stack<Object>();//入栈

Stack<Object> s2 = new Stack<Object>();//出栈

// Push element x to the back of queue.
public void push(Object x) {
s1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if (!s2.isEmpty()) {
s2.pop();
} else {
while (!s1.isEmpty()) {
s2.push(s1.pop());
}
s2.pop();
}
}

// Get the front element.
public Object peek() {
if (!s2.isEmpty()) {
return s2.peek();
} else {
while (!s1.isEmpty()) {
s2.push(s1.pop());
}
return s2.peek();
}
}

// Return whether the queue is empty.
public boolean empty() {
return s1.empty() && s2.empty();
}
//測试
public static void main(String[] args) {
MyQueue queue = new MyQueue();
queue.push(1);
queue.pop();
System.out.println(queue.empty());
}
}