17 Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

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

Solution

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> letter(8, "");
        letter[0] = "abc";
        letter[1] = "def";
        letter[2] = "ghi";
        letter[3] = "jkl";
        letter[4] = "mno";
        letter[5] = "pqrs";
        letter[6] = "tuv";
        letter[7] = "wxyz";
        vector<string> res1;
        if ( digits.size() == 0 ) return res1;
        res1.push_back("");
        for ( int i = 0; i <= digits.size()-1; i++ ) {
            vector<string> res2;
            int nres = res1.size();
            int digit = atoi(digits.substr(i, 1).c_str())-2;
            for ( int j = 0; j <= nres-1; j++ ) {
                string tmp = res1[j];
                for ( int k = 0; k <= letter[digit].size()-1; k++ ) res2.push_back(tmp + letter[digit][k]);
            }
            res1 = res2;
        }
        return res1;
    }
};