Letter Combinations of a Phone Number Given a digit string, return all possible letter combinations that the number could represent.

letterCombinations.cc

class Solution {
public:
    
    void _lComb(int idx, int idxStr, const string & digits, string & str, vector<string> & res)
    {
        static string dMap[10] = {" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        if(idx == digits.length())
        {
            res.push_back(str);
        }else
        {
            int d = digits[idx] - '0';
            if(d != 1)
            {
                string comb = dMap[d];
                for(int i = 0; i<(int)comb.length(); ++i)
                {
                    str[idxStr] = comb[i];
                    _lComb(idx+1, idxStr+1, digits, str, res);
                }
            }else
            {
                _lComb(idx+1, idxStr, digits, str, res);
            }
        }
    }
    
    vector<string> letterCombinations(string digits) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        string str(digits.length(),'\0');
        vector<string> res;
        _lComb(0, 0, digits, str, res);
        return res;
    }
};