Combsum(recursion)

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
       List<List<Integer>> ans = new ArrayList<>();

        backtrack(0, candidates, target, new ArrayList<>(), ans);

        return ans;
    }
    private void backtrack(int index,
                           int[] candidates,
                           int target,
                           List<Integer> curr,
                           List<List<Integer>> ans) {
        if (target == 0) {
            ans.add(new ArrayList<>(curr));
            return;
        }
        if (target < 0 || index == candidates.length) {
            return;
        }
        curr.add(candidates[index]);

        backtrack(
                index,                    
                candidates,
                target - candidates[index],
                curr,
                ans
        );

        curr.remove(curr.size() - 1);       
        backtrack(
                index + 1,
                candidates,
                target,
                curr,
                ans
        ); 
    }
}