Leetcode 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. Each combination's factors must be sorted ascending, for example: The factors of 2 and 6 is [2, 6], not [6, 2].
2. You may assume that n is always positive.
3. 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]
]

Solution 1: DFS

 public class Solution {  
   public List<List<Integer>> getFactors(int n) {  
     List<Integer> one=new ArrayList<>();  
     List<List<Integer>> res=new ArrayList<>();  
     for (int i=2; i*i<=n; i++) dfs(i,n,one,res);  
     return res;  
   }  
   private void dfs(int d, int n, List<Integer> one, List<List<Integer>> res) {  
     if (n%d!=0) return;  
     one.add(d);  
     n/=d;  
     one.add(n);  
     res.add(new ArrayList<Integer>(one));  
     one.remove(one.size()-1);  
     for (int i=d; i*i<=n; i++) dfs(i,n,one,res);  
     one.remove(one.size()-1);  
   }  
 }