Leetcode:Letter Combinations of a Phone Number 手机键盘字母映射

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        const vector<string> letter = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; // 0,1,2...
        vector<string> result;
        dfs(letter, digits, 0, "", result);
        return result;
        
    }
    
    void dfs(const vector<string>& letter, string& digits, int cur, string path, vector<string>& result) {
        if (cur == digits.size()) {
            result.push_back(path);
            return;
        }
        for (auto c : letter.at(digits.at(cur) - ‘0‘)) {
            dfs(letter, digits, cur + 1, path + c, result);
        }
    }
};