17. Letter Combinations of a Phone Number java solutions

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.

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<String>();
        if("".equals(digits) || digits == null) return ans;
        String[] numTochar = {"", "", "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        Combinations(ans,0,digits,"",numTochar);
        return ans;
    }
    
    public void Combinations(List<String> ans, int s,String digits, String tmp,String[] numTochar){
        if(s == digits.length()){
            ans.add(new String(tmp));
            return;
        }
        //for(int i = s; i < digits.length();i++){第一遍想错了，多了个循环 。一直AC不了
            int index = digits.charAt(s) - '0';
            for(int j = 0; j < numTochar[index].length(); j++){
                Combinations(ans,s+1,digits,tmp + numTochar[index].charAt(j),numTochar);
            }
       // }
    }
}