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 ) ``` function Stack() { this.item = [] this.push = (node) => { return this.item.push(node) } this.pop = () => { return this.item.pop() } this.isEmpty = () => { return this.item.length === 0 } } var stack1 = new Stack() var stack2 = new Stack() function appendTail(node) { stack1.push(node) } function deleteHead() { if(stack1.isEmpty() && stack2.isEmpty()){ return -1 } if(stack2.isEmpty()){ while(!stack1.isEmpty()){ stack2.push(stack1.pop()) } } return stack2.pop() } // appendTail('A') // appendTail('B') // appendTail('C') const res = deleteHead() // const res1 = deleteHead() // const res2 = deleteHead() console.log(res) ```