01用栈实现队列

使用栈实现队列的下列操作：

push(x) -- 将一个元素放入队列的尾部。
pop() -- 从队列首部移除元素。
peek() -- 返回队列首部的元素。
empty() -- 返回队列是否为空。

题目来源：力扣(LeetCode)

1.用一个栈实现

/**
 * Initialize your data structure here.
 */
var MyQueue = function() {
     
    this.stack = [];
};

/**
 * Push element x to the back of queue. 
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function(x) {
     
    this.stack.push(x);
};

/**
 * Removes the element from in front of queue and returns that element.
 * @return {number}
 */
MyQueue.prototype.pop = function() {
     
    return this.stack.shift();
};

/**
 * Get the front element.
 * @return {number}
 */
MyQueue.prototype.peek = function() {
     
    return this.stack[0];
};

/**
 * Returns whether the queue is empty.
 * @return {boolean}
 */
MyQueue.prototype.empty = function() {
     
    return !this.stack.length;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */

2.用两个栈实现，一个用于入队，一个用于出队

/**
 * Initialize your data structure here.
 */
var MyQueue = function() {
     
    this.stack = [];
    this.stack2 = []
};

/**
 * Push element x to the back of queue. 
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function(x) {
     
    this.stack.push(x);
};

/**
 * Removes the element from in front of queue and returns that element.
 * @return {number}
 */
MyQueue.prototype.pop = function() {
     
    if(this.stack2.length) return this.stack2.pop();
    while(this.stack.length) {
     
        this.stack2.push(this.stack.pop());
    }
    return this.stack2.length ? this.stack2.pop() : null;
};

/**
 * Get the front element.
 * @return {number}
 */
MyQueue.prototype.peek = function() {
     
    return !this.stack2.length? this.stack[0] : this.stack2[this.stack2.length-1];
};

/**
 * Returns whether the queue is empty.
 * @return {boolean}
 */
MyQueue.prototype.empty = function() {
     
    return !this.stack.length && !this.stack2.length;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */