《leetCode》：Letter Combinations of a Phone Number

题目：

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) {    List<String> res = new ArrayList<>();    if (digits == null || digits.length() == 0) return res;    StringBuilder sb = new StringBuilder();    combine(res, digits, sb, 0);    return res;    }    private void combine(List<String> res, String digits, StringBuilder sb, int posn) {    if (posn == digits.length()) {    res.add(sb.toString());    return;    }    String letters = KEYS[digits.charAt(posn) - '0'];    for (int i = 0; i < letters.length(); i++) {    int sbLen = sb.length();    combine(res, digits, sb.append(letters.charAt(i)), posn+1);    sb.setLength(sbLen);    }    }}