leetcode18-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

class Solution{    public:    vector<string> letterCombinations(string digits) {       // Start typing your C/C++ solution below       // DO NOT write int main() function      const string letters[] = {"","","abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};  vector<string> ret(1, "");  for(int i=0;i<digits.size();i++){if(digits[i]!='0'&&digits[i]!='1'){int retsize=ret.size();for(int j=0;j<retsize;j++){for(int k=0;k<letters[digits[i]-48].size();k++){if(k==letters[digits[i]-48].size()-1)ret[j]+=letters[digits[i]-48][k];elseret.push_back(ret[j]+letters[digits[i]-48][k]);}}}}return ret;      } };