https://leetcode.com/problems/letter-combinations-of-a-phone-number/

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        map<char, string> keyboard;
        keyboard['2'] = "abc";
        keyboard['3'] = "def";
        keyboard['4'] = "ghi";
        keyboard['5'] = "jkl";
        keyboard['6'] = "mno";
        keyboard['7'] = "pqrs";
        keyboard['8'] = "tuv";
        keyboard['9'] = "wxyz";
        keyboard['*'] = "+";
        keyboard['0'] = " ";
        
        vector<string> rst;
        string sol = "";
        
        if (digits.length() == 0) {
            return rst;
        }
        
        letterCombinations(digits, 0, sol, rst, keyboard);
        
        return rst;
    }
    
    void letterCombinations(const string &digits, int pos, string &sol, vector<string> &rst, map<char, string> &keyboard) {
        if (sol.length() == digits.length()) {
            rst.push_back(sol);
            return;
        }
        
        string candi = keyboard[digits[pos]];
        
        for (int i = 0; i < candi.length(); i++) {
            sol.push_back(candi[i]);
            letterCombinations(digits, pos+1, sol, rst, keyboard);
            sol.pop_back();
        }
    }
};