17. 电话号码的字母组合

1. 题目链接

2. 解题思路：一个简单的回溯题目。

3. 代码

   class Solution {
   public:
   
       map<char, string> table{
           {'2', "abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"}, {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}
       };
   
       // 目前已经在处理digits[i]，path是[0...i]已经收集的字符了
       void process(string &digits, int i, vector<string> &res, string &path) {
           if (i == digits.length()) {
               res.push_back(path);
               return ;
           }
           string tmp = table[digits[i]];  
           for (auto &ch : tmp) {
               path.push_back(ch);
               process(digits, i + 1, res, path);
               path.pop_back();     // 恢复现场
           }
       }
       vector<string> letterCombinations(string digits) {
           vector<string> ans;
           if (digits.length() == 0) {
               return ans;
           }
           string path = "";
           process(digits, 0, ans, path);
           return ans;
       }
   };