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

class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(nums);
        permu(nums,res,new boolean[nums.length],new ArrayList<Integer>(),0);
        return res;
    }
    public void permu(int[] nums,List<List<Integer>> res,boolean[] visited,ArrayList<Integer> cur,int index){
        if(index == nums.length){
            res.add(new ArrayList<Integer>(cur));
            return;
        }
        for(int i = 0,len = nums.length;i<len;i++){
            //避免逆序操作。
            if(visited[i] || (i!=0&&nums[i-1] == nums[i] && !visited[i-1])){
                continue;
            }
            visited[i] = true;
            cur.add(nums[i]);
            permu(nums,res,visited,cur,index+1);
            visited[i] = false;
            cur.remove(cur.size()-1);
        }
    }
}