import java.util.ArrayList;
import java.util.Arrays;

public class Permutations {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3};
        ArrayList<ArrayList<Integer>> result = permute(arr);
        System.out.println(result);
    }

    public static ArrayList<ArrayList<Integer>> permute(int[] nums) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return result;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Arrays.sort(nums);
        boolean[] used = new boolean[nums.length];
        helper(result, list, nums, used);
        return result;
    }

    public static void helper(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> list, int[] nums, boolean[] used) {
        if (list.size() == nums.length) {
            result.add(new ArrayList<Integer>(list));
            return;
        }
        for (int i = 0; i < nums.length; i++) {
            if (used[i] || (i > 0 && nums[i] == nums[i - 1] && !used[i - 1])) {
                continue;
            }
            used[i] = true;
            list.add(nums[i]);
            helper(result, list, nums, used);
            used[i] = false;
            list.remove(list.size() - 1);
        }
    }
}