LeetCode-Letter combinations of a phone number

LeetCode----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();
        }
    }
};