leetcode 17 Letter Combinations of a Phone Number

 1 class Solution {
 2 private:
 3     // 创建映射map
 4     unordered_map<char, string> num2char {{‘2‘, "abc"}, {‘3‘, "def"}, {‘4‘, "ghi"}, {‘5‘, "jkl"},
 5                                           {‘6‘, "mno"}, {‘7‘, "pqrs"}, {‘8‘, "tuv"}, {‘9‘, "wxyz"}};
 6     
 7     void letterCombinations(vector<string> &res, string &combination, string digits, int index, int len) {
 8         if (index == len) {
 9             res.push_back(combination);
10             return;
11         }
12         //遍历第index位可以选择的字符
13         for (char c : num2char[digits[index]]) {
14             combination.push_back(c);
15             letterCombinations(res, combination, digits, index + 1, len);
16             combination.pop_back();
17         } 
18         /*
19         string current = num2char[digits[index]];
20         for (int i = 0; i < current.length(); i++) {
21             combination.push_back(current[i]);
22             letterCombinations(res, combination, digits, index + 1, len);
23             combination.pop_back();
24         } */
25     }
26 public:
27     vector<string> letterCombinations(string digits) {
28     
29         int len = digits.length();
30         vector<string> res;
31         if (len == 0) {
32             return res;
33         }
34         
35         string combination;
36         letterCombinations(res, combination, digits, 0, len);
37         return res;
38     }
39 };