254. Factor Combinations

Numbers can be regarded as product of its factors. For example,

8 = 2 x 2 x 2;
  = 2 x 4.
      

Write a function that takes an integer n and return all possible combinations of its factors.

Note: 

1. You may assume that n is always positive.
2. Factors should be greater than 1 and less than n.

Examples: 

input: 

1

output: 

[]
      

input: 

37

output: 

[]
      

input: 

12

output:

[
  [2, 6],
  [2, 2, 3],
  [3, 4]
]
      

input: 

32

output:

[
  [2, 16],
  [2, 2, 8],
  [2, 2, 2, 4],
  [2, 2, 2, 2, 2],
  [2, 4, 4],
  [4, 8]
]
      

public class Solution {
    
	public List<List<Integer>> getFactors(int n) {
		List<List<Integer>> res = new ArrayList<List<Integer>>();
		if (n <= 1) {
			return res;
		}
		List<Integer> list = new ArrayList<Integer>();
		factors(n, list, res, 2);
		return res;
	}

	public void factors(int n, List<Integer> list, List<List<Integer>> res, int start) {
		if (n == 1) {
			if (list.size() > 1) {
				res.add(new ArrayList<Integer>(list));
			}
			return;
		}
		for (int i = start; i <= n; i++) {
			if (n % i == 0) {
				list.add(i);
				factors(n / i, list, res, i);
				list.remove(list.size() - 1);
			}
		}
	}
}
           

i從start開始，而不是每次都從2開始。