import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return result;
        }
        perm(result, nums, 0, nums.length - 1);
        return result;
    }

    public void perm(List<List<Integer>> result, int[] nums, int start, int end) {
        Set<Integer> permed = new HashSet<>();
        if (start > end) {
            List<Integer> list = new ArrayList<>();
            for (int num : nums) {
                list.add(num);
            }
            result.add(list);
        } else {
            for (int i = start; i <= end; i++) {
                if(permed.add(nums[i])) {
                    swap(nums, start, i);
                    perm(result, nums, start + 1, end);
                    swap(nums, start, i);
                }
            }
        }
    }

    public void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }

    public static void main(String[] args) {
        int[] nums = {1,1,2};
        List<List<Integer>> result = new Solution().permute(nums);
        System.out.println(result);
    }
}