Factor Combinations

Source

• leetcode: 254

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 【i】n and return all possible combinations of its factors.
Note: 
Each combination's factors must be sorted ascending, for example: The factors of 2 and 6 is [2, 6], not [6, 2].
You may assume that 【i】n is always positive.
Factors should be greater than 1 and less than 【i】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]]

Java

public List<List<Integer>> getFactors(int n) {
    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){
    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);
        }
    }
}