Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:

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[
  [1,1,2],
  [1,2,1],
  [2,1,1]
]
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public class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> result = new ArrayList<>();
        permutation(result, new ArrayList<Integer>(), nums, new boolean[nums.length]);
        return result;
    }
    
    public void permutation(List<List<Integer>> result, ArrayList<Integer> cur, int[] nums, boolean[] used) {
        if (cur.size() == nums.length) {
            result.add(new ArrayList<Integer>(cur));
        } else {
            for (int i = 0; i < nums.length; i++) {
                if (used[i] || (i > 0 && nums[i] == nums[i - 1] && !used[i - 1])) continue;
                cur.add(nums[i]);
                used[i] = true;
                permutation(result, cur, nums, used);
                cur.remove(cur.size() - 1);
                used[i] = false;
            }
        }
    }
}