Letter combinations of a phone number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

分析：

典型的 DFS

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> dict(10), res;        dict[2] = "abc";        dict[3] = "def";        dict[4] = "ghi";        dict[5] = "jkl";        dict[6] = "mno";        dict[7] = "pqrs";        dict[8] = "tuv";        dict[9] = "wxyz";        string path;        DFS(0, digits, path, dict, res);        return res;    }private:    void DFS(int cur, string& digits, string& path, vector<string> &dict, vector<string>&res)    {        // exit        if(cur == digits.size()){            res.push_back(path);            return;        }                int index = digits[cur] - '0';        for(int i=0; i<dict[index].size(); ++i)        {            path.push_back(dict[index][i]);            DFS(cur+1, digits, path, dict, res);            path.pop_back();        }    }};