https://leetcode-cn.com/problems/implement-queue-using-stacks/

    1. /**
    2.  * Your MyQueue object will be instantiated and called as such:
    3.  * var obj = new MyQueue()
    4.  * obj.push(x)
    5.  * var param_2 = obj.pop()
    6.  * var param_3 = obj.peek()
    7.  * var param_4 = obj.empty()
    8.  */
    9. class MyQueue {
    10.     private stack1: number[] = []
    11.     private stack2: number[] = []
    13.   constructor() {
    15.   }
    17.   push(x: number): void {
    18.     this.stack1.push(x)
    19.   }
    21.   //  从队列的开头移除并返回元素
    22.   pop(): number|undefined {
    23.     let res;
    24.     // 将 stack 移动到 stack2
    25.     while (this.stack1.length){
    26.       const n = this.stack1.pop()
    27.       if (n!== undefined)this.stack2.push(n)
    28.     }
    29.     res = this.stack2.pop()
    30.     while (this.stack2.length){
    31.       const n = this.stack2.pop()
    32.       if (n!== undefined)this.stack1.push(n)
    33.     }
    34.     return  res || undefined
    35.   }
    37.   peek(): number | undefined {
    38.     if (this.stack1.length > 0) {
    39.       return this.stack1[0]
    40.     }
    41.   }
    43.   empty(): boolean {
    44.     return this.stack1.length === 0
    45.   }
    46. }