Given a collection of numbers that might contain duplicates, return all possible unique permutations.
For example,
[1,1,2] have the following unique permutations:
[
[1,1,2],
[1,2,1],
[2,1,1]
]

public class Solution {
    private List<List<Integer>> ans = new ArrayList<List<Integer>>();
    private Set<ArrayList<Integer>> currentSet = new HashSet<ArrayList<Integer>>();
    public List<List<Integer>> permuteUnique(int[] nums) {
        Arrays.sort(nums);
      //  List<Integer> result = new ArrayList<Integer>();
        if(nums.length<=0||nums == null)
           return ans;
        dfs(0,nums);
        ans = new ArrayList<List<Integer>>(currentSet);
        return ans;
    }
    
    void dfs(int start,int[] nums)
    {
        if(start == nums.length)
        {
            ArrayList<Integer> temp = new ArrayList<Integer>();
            for(int i=0;i<nums.length;i++)
               temp.add(nums[i]);
            
            currentSet.add(temp);
          //  ans.add(temp);
        }
        else
        {
           for(int j=start;j<nums.length;j++)
           {
               if(j!=start&&nums[j]==nums[j-1])
                  continue;
               int temp1 = nums[start];
               nums[start] = nums[j];
               nums[j] = temp1;
               
               dfs(start+1,nums);
               
               int temp2 = nums[start];
               nums[start] = nums[j];
               nums[j] = temp2;
               
           }
        }
    }
}