254. Factor Combinations

https://leetcode.com/problems/factor-combinations/description/

 

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]
]

 

 

 

Sol:

 

public class Solution {
    public List<List<Integer>> getFactors(int n) {
        // recursive DFS
        
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        helper(result, new ArrayList<Integer>(), n, 2);
        return result;
        
    }
    
    public void helper(List<List<Integer>> result, List<Integer> item, int n, int start){
        // base case
        if (n <= 1){
            if (item.size() > 1){
                result.add(new ArrayList<Integer>(item));
            }
            return ;
        }
        
        for (int i = start; i <= n; ++i){
            if (n % i == 0){
                item.add(i);
                helper(result, item, n/i, i);
                item.remove(item.size() - 1);
            }
        }
        
        
    }
        
        
        
}