Factor Combinations

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

public List<List<Integer>> getFactors(int n) {
    List<List<Integer>> result = new ArrayList();
    if (n <= 3) return result;
    helper(n, -1, result, new ArrayList<Integer>());
    return result;
}

// Because the largest factor is no more than sqaure root of the number 
private void helper(int n, int lower, List<List<Integer>> result, List<Integer> current) {
    if (lower != -1) {
        current.add(n);
        result.add(new ArrayList<Integer>(current));
        current.remove(current.size() - 1);
    }

    int upper = (int)Math.sqrt(n);
    for (int i = Math.max(2, lower); i <= upper; i++) {
        // if i is a factor
        if (n % i == 0) {
            current.add(i);
            helper(n / i, i, result, current);
            current.remove(current.size() - 1);
        }
    }
}