public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(nums==null)
        return res;
        backtrack(res, new ArrayList<>(),nums);
        return res;
    }
    public void backtrack(List<List<Integer>> res, ArrayList<Integer> tempList, int[] nums){
        if(tempList.size()==nums.length){
            res.add(new ArrayList(tempList));
        }
        for(int i=0;i<nums.length;i++){
            if(tempList.contains(nums[i])) continue;
//skip dupulicate
            else{
             tempList.add(nums[i]);
             backtrack(res,tempList,nums);
             tempList.remove(tempList.size()-1);
//Back one
            }
        }
    }