队列实现栈

function Stack() {
  this.queue = []
}

// 入栈
Stack.prototype.push = function (item) {
  this.queue.push(item)
}

// 出栈
Stack.prototype.pop = function () {
  if (this.queue.length === 0) return null
  const top = this.queue.unshift()
  this.queue.splice(this.queue.length - 1, 1)
  return top
}

// 查看栈顶元素
Stack.prototype.peek = function () {
  if (this.queue.length === 0) return null
  return this.queue[this.queue.length - 1]
}

// 判断栈是否为空
Stack.prototype.isEmpty = function () {
  return this.queue.length === 0
}

// 栈是先进后出,后进先出
const stack = new Stack()
stack.push(1)
stack.push(2)
stack.push(3)

console.log(stack.pop()) // 3
console.log(stack.peek()) // 2
console.log(stack.isEmpty()) // false