[LeetCode] 254. Factor Combinations

Problem

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:

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

Input: 1
Output: []

Example 2:

Input: 37
Output:[]

Example 3:

Input: 12
Output:
[
  [2, 6],
  [2, 2, 3],
  [3, 4]
]

Example 4:

Input: 32
Output:
[
  [2, 16],
  [2, 2, 8],
  [2, 2, 2, 4],
  [2, 2, 2, 2, 2],
  [2, 4, 4],
  [4, 8]
]

Solution

class Solution {
    public List<List<Integer>> getFactors(int n) {
        List<List<Integer>> res = new ArrayList<>();
        if (n == 1) return res;
        helper(n, 2, new ArrayList<>(), res);
        return res;
    }
    private void helper(int n, int factor, List<Integer> temp, List<List<Integer>> res) {
        if (n == 1 && temp.size() > 1) {
            res.add(new ArrayList<>(temp));
        }
        //i starts from factor, to ensure that all factors equals or larger than 2
        for (int i = factor; i <= n; i++) {
            if (n%i == 0) {
                temp.add(i);
                helper(n/i, i, temp, res);
                temp.remove(temp.size()-1);
            }
        }
    }
}