[leetcode 17] 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.

class Solution {public:    vector<string> keyboard{" ", "","abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};    vector<string> letterCombinations(string digits) {        vector<string> res;        dfs(digits, 0, "", res);        return res;    }    void dfs(string digits, int cur, string path, vector<string> &res) {        if (cur == digits.size()) {            res.push_back(path);            return ;        }        for (auto c : keyboard[digits[cur]-'0']) {            dfs(digits, cur+1, path+c, res);        }    }};