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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want

Code:

class Solution {
public:
    int len;
    void combine(int n, string s, vector<string> &res){
        if(n==len)
            res.push_back(s);
        else
            for(int i=0;i<res[n].size();i++)
                combine(n+1,s+res[n][i],res);
    }
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        len=digits.size();
        string letters[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        for(int i=0;i<len;i++){
            if(digits[i]!='0'&&digits[i]!='1')
                res.push_back(letters[digits[i]-'2']);
        }
        combine(0,"",res);
        res.erase(res.begin(),res.begin()+len);
        return res;
    }
};

Another answer:

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        const string letters[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

        vector<string> ret(1, "");
        for (int i = 0; i < digits.size(); ++i) {
            for (int j = ret.size() - 1; j >= 0; --j) {
                const string &s = letters[digits[i] - '2'];
                for (int k = s.size() - 1; k >= 0; --k) {
                    if (k)
                        ret.push_back(ret[j] + s[k]);
                    else
                        ret[j] += s[k];
                }
            }
        }

        return ret;
    }
};