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docs: add No204、No581题解
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‎leetcode刷题/README.md

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- [No.160 相交链表](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No160_get-intersection-node.md)
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- [No.169 多数元素](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No169_majority-element.md)
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- [No.189 旋转数组](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No189_rotate-arr.md)
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- [No.204 计数质数](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No204_count-primes.md)
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- [No.206 反转链表](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No206_reverse-list.md)
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- [No.234 回文链表](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No234_is-palindrome.md)
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- [No.237 删除链表节点](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No237_delete-node.md)
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- [No.412 Fizz Buzz](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No412_fizz-buzz.md)
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- [No.448 找到所有数组中消失的数字](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No448_find-disappeared-numbers.md)
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- [No.492 构造矩形](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No492_construct-rectangle.md)
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- [No.581 最短无序连续子数组](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No581_find-unsorted-subarray.md)
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## 分类
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- [No.189 旋转数组](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No189_rotate-arr.md)
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- [No.283 移动零](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No283_move-zeroes.md)
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- [No.350 两个数组的交集 II](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No350_intersect.md)
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- [No.448 找到所有数组中消失的数字](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No448_find-disappeared-numbers.md)
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- [No.581 最短无序连续子数组](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No581_find-unsorted-subarray.md)
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### 字符串
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#### 数学初级
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- [No.326 3的幂](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No326_is-power-of-three.md)
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- [No.326 3的幂](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No326_is-power-of-three.md)
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- [No.204 计数质数](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No204_count-primes.md)
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- [No.492 构造矩形](https://github.com/Mayandev/javascript_algorithm/blob/master/leetcode%E5%88%B7%E9%A2%98/note/No492_construct-rectangle.md)
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# No.204 计数质数
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难度:`easy`
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统计所有小于非负整数 n 的质数的数量。
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## 示例
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示例:
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```
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输入: 10
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输出: 4
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解释: 小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
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```
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## 解题思路
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使用暴力法,一次判断2-n中质数的个数。
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这里使用排除法,例如循环到 2 的时候,将所有2的倍数的数排除,以此类推。
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代码如下:
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```javascript
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var countPrimes = function(n) {
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let count = 0;
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let signs = new Uint8Array(n);
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for (let i = 2; i < n; i++) {
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if (!signs[i - 1]) {
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count++;
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for (let j = i * i; j <= n; j += i) {
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signs[j - 1] = true;
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}
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}
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}
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return count;
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};
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```
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# No.581 最短无序连续子数组
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难度:`easy`
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给定一个整数数组,你需要寻找一个连续的子数组,如果对这个子数组进行升序排序,那么整个数组都会变为升序排序。
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你找到的子数组应是最短的,请输出它的长度。
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## 示例
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示例 1:
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输入: [2, 6, 4, 8, 10, 9, 15]
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输出: 5
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解释: 你只需要对 [6, 4, 8, 10, 9] 进行升序排序,那么整个表都会变为升序排序。
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说明 :
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输入的数组长度范围在 [1, 10,000]。
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输入的数组可能包含重复元素 ,所以升序的意思是<=。
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## 解题思路
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复制一份数组,将数组排序,然后比较两个数组,记录不同值的 index。
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代码如下:
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```javascript
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/**
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* @param {number[]} nums
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* @return {number}
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*/
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var findUnsortedSubarray = function(nums) {
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let arr = [...nums];
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let firstIdx = lastIdx = 0;
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nums.sort(function(a,b){return a-b});
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for (let i = 0, n = nums.length; i < n; i++) {
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if (arr[i] != nums[i]) {
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firstIdx = i;
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break;
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}
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}
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for (let i = nums.length-1; i > 0; i--) {
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if (arr[i] != nums[i]) {
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lastIdx = i;
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break;
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}
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}
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return lastIdx == firstIdx ? 0 : lastIdx-firstIdx +1;
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};
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```

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