17. 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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

public class Solution {    private static final String[] KEYS = {"", "","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    public List<String> letterCombinations(String digits) {        int n = digits.length();        List<String> result = new LinkedList<String>();        if (n==0) return result;        combination("",digits,0,result);        return result;    }    private void combination(String prefix, String digits, int offset, List<String> result){        if(offset>=digits.length()){            result.add(prefix);            return;        }        String letters = KEYS[(digits.charAt(offset)-'0')];        for(int i=0;i<letters.length();i++){            combination(prefix+letters.charAt(i),digits, offset+1,result);        }    }}