docs: add _39_combinationSum

Author: wangpeng

README.md

@@ -3,19 +3,19 @@
 [![996.icu](https://img.shields.io/badge/link-996.icu-red.svg)](https://996.icu)
 
 ## 说明
-- leetcode练习，坚持每天一道，目前已完成161道
+- leetcode练习，坚持每天一道，目前已完成162道
 - 解题语言是Java
 - 每道题都是可编译运行的
 - 每道题有自己的方法和他人优秀解法
 - 每道题会尽量分析一下解题步骤和复杂度
 - 欢迎star、fork、交流，一起互勉
 - 微信号：pp_hdsny（备注leetcode）
 - 网址：https://leetcode-cn.com/
-## 20190520-20190526待解题目列表
+## 本周待解题目列表
 
 回溯算法
 
-- [ ] [39. 组合总和 - Medium](https://leetcode-cn.com/problems/combination-sum/)
+- [x] [39. 组合总和 - Medium](https://leetcode-cn.com/problems/combination-sum/)
 
 - [ ]  [46. 全排列 - Medium](https://leetcode-cn.com/problems/permutations/)
 
@@ -58,7 +58,7 @@
 - [线段树](https://leetcode-cn.com/tag/segment-tree/)（9）
 - [二叉搜索树](https://leetcode-cn.com/tag/binary-search-tree/)（15）
 
-### 题目列表（更新中--已完成161）
+### 题目列表（更新中--已完成162）
 
 | No    | 题目                                                         | 解决方案                                                     | 相关话题                                                     | 难度   | remark                                                       |
 | ----- | ------------------------------------------------------------ | ------------------------------------------------------------ | ------------------------------------------------------------ | ------ | ------------------------------------------------------------ |
@@ -100,6 +100,7 @@
 | #36   | [有效的数独](https://leetcode-cn.com/problems/valid-sudoku/) | [IsValidSudoku](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_36_isValidSudoku.java) | [哈希表](<https://leetcode-cn.com/tag/hash-table/>)          | Medium |                                                              |
 | #37   | [解数独](https://leetcode-cn.com/problems/sudoku-solver/)    | [SolveSudoku](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_37_solveSudoku.java) | [哈希表](<https://leetcode-cn.com/tag/hash-table/>)、[回溯算法](<https://leetcode-cn.com/tag/backtracking/>) | Hard   |                                                              |
 | #38   | [报数](https://leetcode-cn.com/problems/count-and-say/)      | [CountAndSay](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_38_countAndSay.java) | [字符串](<https://leetcode-cn.com/tag/string/>)              | Easy   |                                                              |
+| #39   | [组合总和](https://leetcode-cn.com/problems/combination-sum/) | [CombinationSum](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_39_combinationSum.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[回溯算法](<https://leetcode-cn.com/tag/backtracking/>) | Medium |                                                              |
 | #40   | [组合总和 II](https://leetcode-cn.com/problems/combination-sum-ii/) | [CombinationSum2](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_40_combinationSum2.java) | [数组](<https://leetcode-cn.com/tag/array/>)、[回溯算法](<https://leetcode-cn.com/tag/backtracking/>) | Medium |                                                              |
 | #42   | [接雨水](https://leetcode-cn.com/problems/trapping-rain-water/) | [Trap](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_42_trap.java) | [栈](https://leetcode-cn.com/tag/stack/)、[数组](<https://leetcode-cn.com/tag/array/>)、[双指针](<https://leetcode-cn.com/tag/two-pointers/>) | Hard   |                                                              |
 | #43   | [字符串相乘](https://leetcode-cn.com/problems/multiply-strings/) | [Multiply](https://github.com/pphdsny/Leetcode-Java/blob/master/src/pp/arithmetic/leetcode/_43_multiply.java) | [数学](<https://leetcode-cn.com/tag/math/>)、[字符串](<https://leetcode-cn.com/tag/string/>) | Medium |                                                              |