[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"].

模拟！

class Solution {
public:
    string key[10];
    void initKey() {
        key[0] = " ";
        key[1] = "";
        key[2] = "abc";
        key[3] = "def";
        key[4] = "ghi";
        key[5] = "jkl";
        key[6] = "mno";
        key[7] = "pqrs";
        key[8] = "tuv";
        key[9] = "wxyz";
    }
    void findNext(vector<string> &res, string &digits, string s, int idx) {
        if (s.length() == digits.length()) {
            res.push_back(s);
            return;
        }
        int k = digits[idx] - '0';
        for (int i = 0; i < key[k].length(); ++i) {
            findNext(res, digits, s + key[k][i], idx + 1);
        }
    }
    vector<string> letterCombinations(string digits) {
        initKey();
        vector<string> res;
        string s;
        findNext(res, digits, s, 0);
        return res;
    }
};