文章目录
代码随想录
232.用栈实现队列
https://leetcode.cn/problems/implement-queue-using-stacks/
var MyQueue = function () {
// 用栈模拟队列，需要输入栈和输出栈
this.stackIn = []
this.stackOut = []
};
/**
* @param {number} x
* @return {void}
*/
MyQueue.prototype.push = function (x) {
this.stackIn.push(x)
};
/**
* @return {number}
*/
MyQueue.prototype.pop = function () {
let temp;
if (this.stackOut.length) {
return this.stackOut.pop();
}
// out数组为空
while (this.stackIn.length) {
this.stackOut.push(this.stackIn.pop());
}
return this.stackOut.pop();
};
/**
* @return {number}
*/
MyQueue.prototype.peek = function () {
// 这里是this.pop而非out.pop()
// 虽然pop是从out开始的，但out可能为空
let temp = this.pop();
this.stackOut.push(temp);
return temp;
};
/**
* @return {boolean}
*/
MyQueue.prototype.empty = function () {
if (this.stackIn.length === 0 && this.stackOut.length === 0) return true;
return false;
};
/**
* 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
https://leetcode.cn/problems/implement-stack-using-queues/
两个队列，一个用于备份。
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 () {
// q1为空时，交换两个队列
if (this.queue1.length === 0) {
[this.queue1, this.queue2] = [this.queue2, this.queue1]
}
while (this.queue1.length > 1) {
this.queue2.push(this.queue1.shift())
}
// q1只剩一个
return this.queue1.shift()
};
/**
* @return {number}
*/
MyStack.prototype.top = function () {
let temp = this.pop();
this.queue1.push(temp);
return temp;
};
/**
* @return {boolean}
*/
MyStack.prototype.empty = function () {
if (this.queue1.length === 0 && this.queue2.length === 0) return true;
return false;
};
/**
* 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
https://leetcode.cn/problems/valid-parentheses/
/**
* @param {string} s
* @return {boolean}
*/
var isLeft = function (s) {
if (s === '(') return true;
else if (s === '[') return true;
else if (s === '{') return true;
else return false;
}
var solve = function (l, r) {
if (l === '[' && r === ']') return true;
else if (l === '{' && r === '}') return true;
else if (l === '(' && r === ')') return true;
return false;
}
var isValid = function (s) {
let arr = [];
for (let i = 0; i < s.length; i++) {
if (isLeft(s[i])) {
arr.push(s[i]);
} else {
if (arr.length === 0) return false;
else {
let l = arr.pop();
if (!solve(l, s[i])) return false;
}
}
}
if (arr.length === 0) return true;
return false;
};
1047. 删除字符串中的所有相邻重复项
https://leetcode.cn/problems/remove-all-adjacent-duplicates-in-string/
/**
* @param {string} s
* @return {string}
*/
var removeDuplicates = function (s) {
let arr = [];
for (let i = 0; i < s.length; i++) {
// 出栈
if (arr.length && s[i] === arr[arr.length - 1]) {
arr.pop();
} else {
arr.push(s[i]);
}
}
return arr.join('');
};
150. 逆波兰表达式求值
https://leetcode.cn/problems/evaluate-reverse-polish-notation/
注意：除法要向零取整。也就是：正数向下取整（Math.floor()），负数向上取整（Math.ceil()）。
/**
* @param {string[]} tokens
* @return {number}
*/
var evalRPN = function (tokens) {
let arr = []
for (const token of tokens) {
// 是否不是数字——是符号
if (isNaN(Number(token))) {
let num1 = arr.pop();
let num2 = arr.pop();
let ans;
switch (token) {
case '+': {
ans = num1 + num2;
break;
}
case '*': {
ans = num1 * num2;
break;
}
case '-': {
ans = num2 - num1;
break;
}
case '/': {
// 注意:向零取整
if (num2 * num1 > 0) {
// floor向下取整
ans = Math.floor(num2 / num1);
} else {
// ceil向上取整
ans = Math.ceil(num2 / num1);
}
break;
}
}
arr.push(ans);
// console.log(ans)
} else {
// 数字
arr.push(Number(token));
}
}
return arr[0];
};
239. 滑动窗口最大值（困难-还没写）
https://leetcode.cn/problems/sliding-window-maximum/
347.前 K 个高频元素
https://leetcode.cn/problems/top-k-frequent-elements/
/**
* @param {number[]} nums
* @param {number} k
* @return {number[]}
*/
var topKFrequent = function (nums, k) {
let map = new Map();
for (const num of nums) {
map.set(num, (map.get(num) || 0) + 1);
}
let arr = Array.from(map);
// 对值排序：从大到小
arr.sort((a, b) => (b[1] - a[1]))
let ans = []
for (let i = 0; i < k; i++) {
ans.push(arr[i][0])
}
return ans;
};