LeetCode-232. Implement Queue Using Stacks

问题描述

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

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

Constraints:

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

解答

/*
 * @Description: 232. Implement Queue using Stacks
 * @Author: libk
 * @Github: https://github.com/libk
 */
/** 
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */
const MyQueue = function () {
  this.inStack = []
  this.outStack = []
}

/**
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function (x) {
  this.inStack.push(x)
}

/**
 * @return {number}
 */
MyQueue.prototype.pop = function () {
  if (!this.outStack.length) {
    while (this.inStack.length) {
      this.outStack.push(this.inStack.pop())
    }
  }
  return this.outStack.pop()
}

/**
 * @return {number}
 */
MyQueue.prototype.peek = function () {
  if (!this.outStack.length) {
    while (this.inStack.length) {
      this.outStack.push(this.inStack.pop())
    }
  }
  return this.outStack[this.outStack.length - 1]
}

/**
 * @return {boolean}
 */
MyQueue.prototype.empty = function () {
  return !this.outStack.length && !this.inStack.length
}