第三篇，factor combinations

 1 public class Solution {
 2     public List<List<Integer>> getFactors(int n) {
 3         List<List<Integer>> result = new ArrayList<List<Integer>>();
 4         if (n <= 1) {
 5             return result;//负数如何处理，返回值还是抛异常与面试官讨论
 6         }
 7         List<Integer> path = new ArrayList<Integer>();
 8         findFactor(n, 2, result, path);
 9         return result;
10     }
11     
12     //Fuction signature 写的好是加分项
13     //此处定义private以及命名为findFactor比直接使用public helper更严谨
14     private void findFactor(int n, int currFactor, List<List<Integer>> result, List<Integer> path) {
15         //base case, find a group of solution
16         if (n == 1 && path.size() > 1) {
17             result.add(new ArrayList(path));//construct new ArrayList for Java
18             return;
19         }
20         for (int i = currFactor; i <= n; i++) { //important to have i <= n for example 4 = 2 * 2
21             if (n % i == 0) {
22                 path.add(i);
23                 findFactor(n / i, i, result, path);
24                 path.remove(path.size() - 1);
25             }
26         }
27     }
28 }