Letter Combinations of a Phone Number

public class Solution {

    String[] keybd ={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

    // http://www.cnblogs.com/springfor/p/3879829.html
    public ArrayList<String> letterCombinations(String digits) {

        ArrayList<String> res = new ArrayList<String>();

        if(digits.length()<1) return res;

        StringBuilder item = new StringBuilder();

        dfs(digits, item, res, 0);

        return res;

    }

    

    public void dfs(String digits, StringBuilder item, ArrayList<String> res, int start){

        if(start== digits.length()) {

            res.add(item.toString()); // 不是 new StringBuilder (item)

            return;

        }

        int num = digits.charAt(start)-'0';

        for(int i=0;i<keybd[num].length();i++){

            item.append(keybd[num].charAt(i));

            dfs(digits, item, res, start+1);

            item.deleteCharAt(item.length()-1);

            

        }

    }

}