[?*]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 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;
}
}