LeetCode-225. Implement Stack Using Queues

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问题描述

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

  • You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
  • Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.

Example :

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Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, top, and empty.
  • All the calls to pop and top are valid.

解答

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/*
 * @Description: 225. Implement Stack using Queues
 * @Author: libk
 * @Github: https://github.com/libk
 */
/** 
 * Your MyStack object will be instantiated and called as such:
 * var obj = new MyStack()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.empty()
 */

const MyStack = function () {
  this.queue = []
}

/**
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function (x) {
  let length = this.queue.length
  this.queue.push(x)
  for (let i = 0; i < length; i++) {
    this.queue.push(this.queue.shift())
  }
}

/**
 * @return {number}
 */
MyStack.prototype.pop = function () {
  return this.queue.shift()
}

/**
 * @return {number}
 */
MyStack.prototype.top = function () {
  return this.queue[0]
}

/**
 * @return {boolean}
 */
MyStack.prototype.empty = function () {
  return !this.queue.length
}