add C++ solution2

solutions/17. Letter Combinations of a Phone Number.md

@@ -53,5 +53,26 @@ public:
 ```
 
 ## 思路二
-见[此处](https://leetcode.com/problems/letter-combinations-of-a-phone-number/discuss/8454/My-C%2B%2B-solution-use-DFS).   
-有时间了再自己实现一下。
+``` C++
+class Solution {
+private:
+    const vector<string>digit2char{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
+    void DFS(const string &digits, string out, vector<string> &res){
+        int start = out.size();
+        if(start == digits.size()){
+            res.push_back(out);
+            return;
+        }
+        for(char c: digit2char[digits[start]-'0'])
+            DFS(digits, out+c, res);
+    }
+public:
+    vector<string> letterCombinations(string digits) {
+        vector<string>res;
+        if(digits.size() == 0) return res;
+        string out;
+        DFS(digits, out, res);
+        return res;
+    }
+};
+```