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];
					else
						ret.push_back(ret[j]+letters[digits[i]-48][k]);
				}
			}
		}
	}
	return ret;  
    } 
};