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"].

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        String[] str= {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        ArrayList<String> res = new ArrayList<String>();
        StringBuilder sb  = new StringBuilder ();
        DFS(digits,str,0,sb,res);
        return res;
    }
    public void DFS(String digits,String[]str,int start,StringBuilder sb,ArrayList<String>res){
        if(start==digits.length()){
            res.add(sb.toString());
            return;
        }
        int d = digits.charAt(start)-'0';
        for(int i=0;i<str[d].length();i++){
            sb.append(str[d].charAt(i));
            DFS(digits,str,start+1,sb,res);
            sb.deleteCharAt(sb.length()-1);
        }
    }
}

 start  看成是第几个数字