Letter Combination of 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.

 string keypad[8]  = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

void generate(string digits, string result, int i,vector<string> &ans)
{

 if(i >= digits.size())
 {
  ans.push_back(result);
  return;
 }


 if(digits[i] <= '1' ||  digits[i] > '9')
   {
       generate(digits, result, i +1, ans);
   }
   else
   {
    string t  = keypad[digits[i] - '2'];
    for(int r  = 0; r < t.size(); r++)
    {
     generate(digits, result + t[r], i+1, ans);
    }
   }
}

vector<string> letterCombinations(string digits) {
  string result = "";      
  vector<string> ans;
  generate(digits, result, 0, ans);
  return ans;
 }