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：暴力搜索所有结果。

代码如下：

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