【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.

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        mapping = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> ret;
        letterCombinationsCore(digits, "", ret);
        return ret;
    }
    void letterCombinationsCore(string digits, string sub, vector<string> &ret) {
        if (sub.size() == digits.size()) {
            ret.push_back(sub);
        } else {
            int i = digits[sub.size()] - '0';
            for (int j = 0; j < mapping[i - 2].size(); ++j) {
                sub.push_back(mapping[i - 2][j]);
                letterCombinationsCore(digits, sub, ret);
                sub.pop_back();
            }
        }
    }
    vector<string> mapping;
};

又是一个递归搜索题。