1 var arr = [
2 [],
3 [],
4 ['a', 'b', 'c'],
5 ['d', 'e', 'f'],
6 ['g', 'h', 'i'],
7 ['j', 'k', 'l'],
8 ['m', 'n', 'o'],
9 ['p', 'q', 'r', 's'],
10 ['t', 'u', 'v'],
11 ['w', 'x', 'y', 'z']
12 ];
13
14 var letterCombinations = function(digits) {
15 if (digits.length < 1) {
16 return [];
17 }
18
19 if (digits.length === 1) {
20 return arr[digits[0]];
21 }
22
23 var ret = [],
24 listNow = arr[digits[0]],
25 listBehind = letterCombinations(digits.substring(1)),
26 lenBehind = listBehind.length;
27
28 for (var i = 0; i < listNow.length; i++) {
29 for (var j = 0; j < listBehind.length; j++) {
30 ret[i * lenBehind + j] = listNow[i] + listBehind[j];
31 }
32 }
33
34 return ret;
35 };
