Binary Tree ZigZag Level Order Traversal leetcode java

 1     public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
 2         ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
 3         
 4         if(root==null)
 5             return res;
 6         
 7         LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
 8         queue.add(root);
 9         
10         int num = 0;
11         boolean reverse = false;//a flag
12         
13         while(!queue.isEmpty()){
14             num = queue.size();
15             ArrayList<Integer> levelres = new ArrayList<Integer>();
16             
17             for(int i = 0; i<num; i++){
18                 TreeNode node = queue.poll();
19                 levelres.add(node.val);
20                 
21                 if(node.left!=null)
22                     queue.add(node.left);
23                 if(node.right!=null)
24                     queue.add(node.right);
25             }
26             
27             if(reverse){
28                 Collections.reverse(levelres);
29                 reverse = false;
30             }else{
31                 reverse = true;
32             }
33             res.add(levelres);
34         }
35         
36         return res;
37     }