Permutations leetcode java

 1     public ArrayList<ArrayList<Integer>> permute(int[] num) {
 2         ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
 3         ArrayList<Integer> item = new ArrayList<Integer>();
 4         
 5         if(num.length==0||num==null)
 6             return res;
 7         boolean[] visited = new boolean[num.length];  
 8         
 9         permutation_helper(num,res,item,visited);
10         return res;
11     }
12     
13     public void permutation_helper(int[] num, ArrayList<ArrayList<Integer>> res, ArrayList<Integer> item,boolean[] visited){
14         if(item.size()==num.length){
15             res.add(new ArrayList<Integer>(item));
16             return;
17         }
18         
19         for(int i = 0; i<num.length;i++){
20             if(!visited[i]){
21                 item.add(num[i]);
22                 visited[i]=true;
23                 permutation_helper(num,res,item,visited);
24                 item.remove(item.size()-1);
25                 visited[i]=false;
26             }
27         }
28     }