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"].

思路：DFS一下就行了

class Solution {
public:
    const string str[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "qprs", "tuv", "wxyz"};       

    void cal(int dep, string &tmp, vector<string> &ans, string &digits) {
        if (dep == digits.size()) {  
            ans.push_back(tmp);  
            return;  
        }  
        
        for (int i = 0; i < str[digits[dep]-'0'].size(); i++) {  
            tmp.push_back(str[digits[dep]-'0'][i]);  
            cal(dep+1, tmp, ans, digits);  
            tmp.pop_back();  
        }  
    }

    vector<string> letterCombinations(string digits) {
        vector<string> ans;
        string tmp = "";
        cal(0, tmp, ans, digits);
        return ans;
    }
};

LeetCode Letter Combinations of a Phone Number