[LeetCode] #39 Combination Sum

Time complexity: O(n!)
because T(n) = nT(n - 1) + k

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        Arrays.sort(candidates);
        helper(result, new ArrayList<Integer>(), 0, candidates, target, 0);
        return result;
    }
    private void helper(List<List<Integer>> result, List<Integer> path, int sum, int[] candidates, int target, int start) {
        if (sum == target) {
            result.add(new ArrayList<Integer>(path));
            return;
        }
        
        for (int i = 0; i < candidates.length; i++) {
            if (start > 1 && start - 1 > i) {
                continue;
            }
            if (sum + candidates[i] <= target) {
                path.add(candidates[i]);
                helper(result, path, sum + candidates[i], candidates, target, i + 1);
                path.remove(path.size() - 1);
            } else {
                break;
            }
        }
    }
}