docs: add _31_nextPermutation

Author: wangpeng

README.md

@@ -21,7 +21,7 @@
 
 - [x] [30. 串联所有单词的子串-Hard](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_30_findSubstring.java)
 
-- [ ]  [31. 下一个排列-Medium](https://leetcode-cn.com/problems/next-permutation/)
+- [x] [31. 下一个排列-Medium](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_31_nextPermutation.java)
 
 - [ ] [32. 最长有效括号-Hard](https://leetcode-cn.com/problems/longest-valid-parentheses/)
 
@@ -94,6 +94,7 @@
 | #28   | [实现strStr()](https://leetcode-cn.com/problems/implement-strstr/) | [StrStr](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_28_strStr.java) | [双指针](<https://leetcode-cn.com/tag/two-pointers/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Easy   |                                                              |
 | #29   | [两数相除](https://leetcode-cn.com/problems/divide-two-integers/) | [Divide](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_29_divide.java) | [数学](<https://leetcode-cn.com/tag/math/>)、[二分查找](<https://leetcode-cn.com/tag/binary-search/>) | Medium |                                                              |
 | #30   | [串联所有单词的子串](https://leetcode-cn.com/problems/substring-with-concatenation-of-all-words/) | [FindSubstring](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_30_findSubstring.java) | [哈希表](<https://leetcode-cn.com/tag/hash-table/>)、[双指针](<https://leetcode-cn.com/tag/two-pointers/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Hard   |                                                              |
+| #31   | [下一个排列](https://leetcode-cn.com/problems/next-permutation/) | [NextPermutation](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_31_nextPermutation.java) | [数组](<https://leetcode-cn.com/tag/array/>)                 | Medium |                                                              |
 | #33   | [搜索旋转排序数组](https://leetcode-cn.com/problems/search-in-rotated-sorted-array/) | [Search](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_33_search.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[二分查找](<https://leetcode-cn.com/tag/binary-search/>) | Medium |                                                              |
 | #34   | [在排序数组中查找元素的第一个和最后一个位置](https://leetcode-cn.com/problems/find-first-and-last-position-of-element-in-sorted-array/) | [SearchRange](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_34_searchRange.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[二分查找](<https://leetcode-cn.com/tag/binary-search/>) | Medium |                                                              |
 | #35   | [搜索插入位置](https://leetcode-cn.com/problems/search-insert-position/) | [SearchInsert](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_35_searchInsert.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[二分查找](<https://leetcode-cn.com/tag/binary-search/>) | Easy   |                                                              |