[*]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"].

gave digits = "23"

i=0 -> result=combine("abc", [""]) ---> [a,b,c];

i=1 -> result=combine("def", [a,b,c]) ---> [ad,bd,cd, ae,be,ce, af,bf,cf];

  public class Solution {
        public static List<String> letterCombinations(String digits) {
            String digitletter[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
            List<String> result = new ArrayList<String>();

            if (digits.length()==0) return result;

            result.add("");
            for (int i=0; i<digits.length(); i++) 
                result = combine(digitletter[digits.charAt(i)-'0'],result);

            return result;
        }

        public static List<String> combine(String digit, List<String> list) {
            List<String> result = new ArrayList<String>();

            for (int i=0; i<digit.length(); i++) 
                for (String x : list) 
                    result.add(x+digit.charAt(i));

            return result;
        }
    }