• 232. 用栈实现队列
• 225. 用队列实现栈
• 20. 有效的括号
• 1047. 删除字符串中的所有相邻重复项
• 150. 逆波兰表达式求值
• 347. 前 K 个高频元素

232. 用栈实现队列

var MyQueue = function () {this.stackIn = []this.stackOut = []
};/** * @param {number} x* @return {void}*/
MyQueue.prototype.push = function (x) {this.stackIn.push(x)
};/*** @return {number}*/
MyQueue.prototype.pop = function () {const size = this.stackOut.lengthif (size) {return this.stackOut.pop()}while (this.stackIn.length) {this.stackOut.push(this.stackIn.pop())}return this.stackOut.pop()};/*** @return {number}*/
MyQueue.prototype.peek = function () {const x = this.pop()this.stackOut.push(x)return x
};/*** @return {boolean}*/
MyQueue.prototype.empty = function () {return !this.stackIn.length && !this.stackOut.length
};/*** 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()*/

225. 用队列实现栈

var MyStack = function () {this.queue1 = []this.queue2 = []
};/** * @param {number} x* @return {void}*/
MyStack.prototype.push = function (x) {this.queue1.push(x)
};/*** @return {number}*/
MyStack.prototype.pop = function () {if (!this.queue1.length) {[this.queue1, this.queue2] = [this.queue2, this.queue1]}while (this.queue1.length>1) {this.queue2.push(this.queue1.shift())}return this.queue1.shift()
};/*** @return {number}*/
MyStack.prototype.top = function () {const x = this.pop()this.queue1.push(x)return x
};/*** @return {boolean}*/
MyStack.prototype.empty = function () {return !this.queue1.length && !this.queue2.length
};/*** 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()*/

20. 有效的括号

/*** @param {string} s* @return {boolean}*/
var isValid = function (s) {const stack = [];let left = "({[";let right = ")}]";for (let ch of s) {if (left.includes(ch)) {stack.push(ch);}if (right.includes(ch)) {let top = stack[stack.length - 1];if (top === undefined) return false;if ((ch === "}" && top === "{") ||(ch === ")" && top === "(") ||(ch === "]" && top === "[")) {stack.pop();}else {return false}}}return stack.length ===0 
};

1047. 删除字符串中的所有相邻重复项

/*** @param {string} s* @return {string}*/
var removeDuplicates = function(s) {const stack = []for(let ch of s){if(stack.length){top = stack[stack.length-1]if(top===ch){stack.pop()}else{stack.push(ch)}}else{stack.push(ch)}}return stack.join('')
};

150. 逆波兰表达式求值

/*** @param {string[]} tokens* @return {number}*/
var evalRPN = function (tokens) {const s = new Map([["+", (a, b) => a * 1 + b * 1],["-", (a, b) => b - a],["*", (a, b) => b * a],["/", (a, b) => (b / a) | 0]]);const stack = [];for (const i of tokens) {if (!s.has(i)) {stack.push(i);continue;}stack.push(s.get(i)(stack.pop(), stack.pop()))}return stack.pop();
};

347. 前 K 个高频元素

/*** @param {number[]} nums* @param {number} k* @return {number[]}*/
var topKFrequent = function (nums, k) {const map = {};for (let ch of nums) {map[ch] === undefined ? (map[ch] = 1) : (map[ch] += 1);}let arr = Object.entries(map);//   注意排序arr.sort((a, b) => b[1] - a[1]);let ans = [];for (let i = 0; i < k; i++) {ans.push(arr[i][0]);}return ans;
};