Leetcode#17 Letter Combinations of a Phone Number

 1 vector<string> res;
 2     
 3 void dfs(string &digits, vector<string> &i2s, string ans, int pos) {
 4   if (pos == digits.length()) {
 5     res.push_back(ans);
 6     return;
 7   }
 8         
 9   for (auto c : i2s[digits[pos] - ‘0‘])
10     dfs(digits, i2s, ans + c, pos + 1);
11 }
12 
13 vector<string> letterCombinations(string digits) {
14   vector<string> i2s {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
15   dfs(digits, i2s, "", 0);
16   return res;
17 }