Leetcode #17: Letter Combinations of a Phone Number

Leetcode_0017.cpp

class Solution {
public:
    map<char, string> numToString = {{'2', "abc"}, {'3', "def"}, {'4', "ghi"}, 
                                     {'5', "jkl"}, {'6', "mno"}, {'7', "pqrs"}, 
                                     {'8', "tuv"}, {'9', "wxyz"}};
    vector<string> answer;
    string input;
    
    void recurse(string path, int index) {
        if (!(index < input.length())) { // the entire string is processed already.
            if (!path.empty()) { // only add non-empty strings.
                answer.push_back(path); // add the current value in final answer.
            }
            return; // nothing else to do here.
        }
        char currentChar = input[index];
        string possibilities = numToString[currentChar];
        for (char c: possibilities) { // for each possibility.
            path.push_back(c); // assume current possibility.
            recurse(path, index+1); // recursively solve the remaining problem.
            path.pop_back(); // undo the current possibility.
        }
    }

    vector<string> letterCombinations(string digits) {
        input = digits;
        recurse("", // empty string in the beginning.
                0);  // what index in input to start the recursion.
        return answer;
    }
};