PermutationII.java

import java.util.*;
class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> list = new ArrayList<>();
        Arrays.sort(nums);
        backtrack(list, new ArrayList<>(), nums, new boolean[nums.length]);
        return list;
    }

    private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, boolean [] used){
        
        if(tempList.size() == nums.length){
            list.add(new ArrayList<>(tempList));
        
        } else{
            for(int i = 0; i < nums.length; i++){
                
                if(used[i] || i > 0 && nums[i] == nums[i-1] && !used[i - 1])                        continue;
        
                used[i] = true; 
                
                tempList.add(nums[i]);
                
                backtrack(list, tempList, nums, used);
                
                used[i] = false; 
                
                tempList.remove(tempList.size() - 1);
            }
        }
    }
}