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);
}
}
}