1+ /*
2+ Author: King, wangjingui@outlook.com
3+ Date: Dec 20, 2014
4+ Problem: Letter Combinations of a Phone Number
5+ Difficulty: Medium
6+ Source: https://oj.leetcode.com/problems/letter-combinations-of-a-phone-number/
7+ Notes:
8+ Given a digit string, return all possible letter combinations that the number could represent.
9+ A mapping of digit to letters (just like on the telephone buttons) is given below.
10+ Input:Digit string "23"
11+ Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
12+ Note:
13+ Although the above answer is in lexicographical order, your answer could be in any order you want.
14+
15+ Solution: ...
16+ */
17+
18+ public class Solution {
19+ public List <String >letterCombinations (String digits ) {
20+ ArrayList <String >res =new ArrayList <String >();
21+ String []keyboard =new String []{" " ,"" ,"abc" ,"def" ,"ghi" ,"jkl" ,"mno" ,"pqrs" ,"tuv" ,"wxyz" };
22+ letterCombinationsRe (keyboard ,res ,digits ,"" );
23+ return res ;
24+ }
25+ public void letterCombinationsRe (String []keyboard ,ArrayList <String >res ,String digits ,String s ) {
26+ if (s .length () ==digits .length ()) {
27+ res .add (s );
28+ return ;
29+ }
30+ String letters =keyboard [digits .charAt (s .length ()) -'0' ];
31+ for (int i =0 ;i <letters .length (); ++i ) {
32+ letterCombinationsRe (keyboard ,res ,digits ,s +letters .charAt (i ));
33+ }
34+ }
35+ }
