用两个栈实现一个队列

题目

请用两个栈，来实现队列的功能，实现功能 add delete length 。

队列 Queue

栈，先进后出

队列，先进先出，API 包括

• add
• delete
• length

class MyQueue {
  stack1 = [];
  stack2 = [];
  add(n) {
    this.stack1.push(n);
  }
  delete() {
    let res;
    const stack1 = this.stack1;
    const stack2 = this.stack2;
    while (stack1.length) {
      const n = stack1.pop();
      if (n != null) {
        stack2.push(n);
      }
    }
    res = stack2.pop();
    while (stack2.length) {
      const n = stack2.pop();
      if (n != null) {
        stack1.push(n);
      }
    }
    return res || null;
  }
  get length() {
    return this.stack1.length;
  }
}