phone_comb.cc

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> mmap({"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"});
        string buf;
        vector<string> result;
        dfs(0, mmap, buf, digits, result);
        return result;
    }
    
    void dfs(int level, vector<string> &mmap, string &buf, string &digits, vector<string> &result) {
        if (level == digits.size()) {
            result.push_back(buf);
            return;
        }
        for (int i = 0; i < mmap[(digits[level] - '0')-2].size(); ++i) {
            buf.push_back(mmap[(digits[level]- '0')-2][i]);
            dfs(level + 1, mmap, buf, digits, result);
            buf.pop_back();
        }
    }
    
};