Given a list of numbers, return all possible permutations.

class Solution {
    /** * @param nums: A list of integers. * @return: A list of permutations. */
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> results = new ArrayList<>();
        if (nums == null) {
            return results;
        }
        if (nums.length == 0) {
            results.add(new ArrayList<Integer>());
            return results;
        }
        List<Integer> result = new ArrayList<>();
        dfsHelper(nums, results, result);
        return results;
    }
    private void dfsHelper(int[] nums,
                           List<List<Integer>> results,
                           List<Integer> result) {
        if (result.size() == nums.length) {
            results.add(new ArrayList<Integer>(result));
        }
        for (int i = 0; i < nums.length; i++) {
            if (result.contains(nums[i])) {
                continue;
            }
            result.add(nums[i]);
            dfsHelper(nums, results, result);
            result.remove(result.size() - 1);
        }
    }
}