implement-queue-using-stacks.js

var MyQueue = function() {
  // 输入栈
  this.stackIn = [];
  // 输出栈
  this.stackOut = [];
};

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

/**
* @return {number}
*/
MyQueue.prototype.pop = function() {
  // [1, 2, 3, 4]
  // []
  // => 
  // []
  // [4, 3, 2, 1]

  // 如果输出栈为空，将输入栈的数据全部导入输出栈
  if (this.stackOut.length === 0) {
    while (this.stackIn.length > 0) {
      this.stackOut.push(this.stackIn.pop());
    }
  }
  return this.stackOut.pop();
};

/**
* @return {number}
*/
MyQueue.prototype.peek = function() {
  // 复用已有方法
  const result = this.pop();
  this.stackOut.push(result);
  return result;
};

/**
* @return {boolean}
*/
MyQueue.prototype.empty = function() {
  return this.stackIn.length === 0 && this.stackOut.length === 0;
};

/**
* 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()
*/