算法练习day9

栈是先进后出，队列是先进先出

一、用栈实现队列

问题要点

栈来模拟队列，需要用到两个栈，输入栈和输出栈，输入的话，往输入栈 push 即可，输出的情况需要判断，如果输出栈有值，则直接 pop，否则，把输入栈的元素全部放到输出栈，然后对输出栈 pop

var MyQueue = function () {
  this.stackIn = [];
  this.stackOut = [];
};

/**
 * 将元素放入队列尾部
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function (x) {
  this.stackIn.push(x);
};

/**
 * 将队列首部移除
 * @return {number}
 */
MyQueue.prototype.pop = function () {
  let size = this.stackOut.length;
  if (size) {
    return this.stackOut.pop();
  }
  while (this.stackIn.length) {
    this.stackOut.push(this.stackIn.pop());
  }
  return this.stackOut.pop();
};

/**
 * 队首元素
 * @return {number}
 */
MyQueue.prototype.peek = function () {
  let val = this.pop();
  this.stackOut.push(val);
  return val;
};

/**
 * @return {boolean}
 */
MyQueue.prototype.empty = function () {
  return !this.stackIn.length && !this.stackOut.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()
 */

二、用队列实现栈

两个队列实现栈

其中第二个队列是备份的作用

var MyStack = function () {
  this.queue1 = [];
  this.queue2 = [];
};

/**
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function (x) {
  this.queue1.push(x);
};

/**
 * @return {number}
 */
MyStack.prototype.pop = function () {
  if (!this.queue1.length) {
    [this.queue1, this.queue2] = [this.queue2, this.queue1];
  }
  while (this.queue1.length > 1) {
    this.queue2.push(this.queue1.shift());
  }
  return this.queue1.shift();
};

/**
 * @return {number}
 */
MyStack.prototype.top = function () {
  let res = this.pop();
  this.queue1.push(res);
  return res;
};

/**
 * @return {boolean}
 */
MyStack.prototype.empty = function () {
  return !this.queue1.length && !this.queue2.length;
};

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

使用一个队列实现

要注意遍历队列的时候留下最后一个，就是需要的栈顶元素

var MyStack = function() {
    this.queue = []
};

/** 
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function(x) {
    this.queue.push(x)
};

/**
 * @return {number}
 */
MyStack.prototype.pop = function() {
    let size = this.queue.length
    while(size-- > 1) {
        this.queue.push(this.queue.shift())
    }
    return this.queue.shift()
};

/**
 * @return {number}
 */
MyStack.prototype.top = function() {
    let res = this.pop()
    this.queue.push(res)
    return res
};

/**
 * @return {boolean}
 */
MyStack.prototype.empty = function() {
    return !this.queue.length
};

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