Letter Combinations of a Phone Number —— Leetcode

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.

这题比较简单，注意细节，我在细节上犯错误了，导致拖了接近半个小时。

class Solution {
public:
    void combines(vector<string> &res, vector<string> &map, string str, string left_digits) {
        if(left_digits == "") {
            res.push_back(str);
			return ;
        }

        char c = left_digits[0];
        string tmp = map[atoi(&c)];
        string leftd = left_digits.erase(0, 1);
        for(int j=0; j<tmp.size(); j++) {
            combines(res, map, str+tmp[j], leftd);
        }
    }

    vector<string> letterCombinations(string digits) {
        vector<string> map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> res;
        if(digits != "")
        	combines(res, map, "", digits);
        return res;
    }
};