public List<List<Integer>> permuteUnique(int[] nums) {

List<List<Integer>> res = new ArrayList<List<Integer>>();

Arrays.sort(nums);

helper(nums, 0, res);

return res;

}

public void helper(int[] nums, int pos, List<List<Integer>> res){

if(pos == nums.length){

List<Integer> list = new ArrayList<Integer>();

for(int n: nums) list.add(n);

res.add(list);

}

HashSet<Integer> set = new HashSet<Integer>();

for(int i = pos; i < nums.length; i++){

if(i!=pos && (nums[i] == nums[i-1] || set.contains(nums[i]))) continue;

set.add(nums[i]);

swap(nums, pos, i);

helper(nums, pos+1, res);

swap(nums, pos, i);

}

}

public void swap(int[] nums, int i, int j){

int temp = nums[i];

nums[i] = nums[j];

nums[j] = temp;

}

}