栈与队列-用两个栈实现队列

剑指 Offer 09. 用两个栈实现队列

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

示例 1：

输入：
["CQueue","appendTail","deleteHead","deleteHead","deleteHead"]
[[],[3],[],[],[]]
输出：[null,null,3,-1,-1]

示例 2：

输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

题解：

var CQueue = function() {
    //入栈
    this.tailStack = [],
    //出栈
    this.headStack = []
};

/** 
 * @param {number} value
 * @return {void}
 */
CQueue.prototype.appendTail = function(value) {
    this.tailStack.push(value)
};

/**
 * @return {number}
 */
CQueue.prototype.deleteHead = function() {
    //出栈中途入栈的情况，判断出栈是否为空，为空时才去读取入栈的数据
    if(!this.headStack.length){
        while(this.tailStack.length){
            this.headStack.push(this.tailStack.pop())
        }
    }

    return this.headStack.pop() ?? -1
};

/**
 * Your CQueue object will be instantiated and called as such:
 * var obj = new CQueue()
 * obj.appendTail(value)
 * var param_2 = obj.deleteHead()
 */

规则：

栈 --> 先进后出 队 --> 先进先出

题目来源：leetcode