46. Permutations

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return res;
        }
        List<Integer> cur = new ArrayList<>();
        helper(nums, cur, res);
        return res;
    }
    
    private void helper(int[] nums, List<Integer> cur, List<List<Integer>> res) {
        if (cur.size() == nums.length) {
            res.add(new ArrayList<>(cur));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (!cur.contains(nums[i])) {
                cur.add(nums[i]);
                helper(nums, cur, res);
                cur.remove(cur.size() - 1);
            }
        }
    }
}