1 public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
 2         ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
 3         if(root == null) return result;
 4         Queue<TreeNode> q = new LinkedList<TreeNode>();
 5         q.offer(root);
 6         ArrayList<Integer> arr = null;
 7         while(!q.isEmpty()){
 8             arr = new ArrayList<Integer>();
 9             int num = q.size();
10             for(int i = 0; i <num; i++) {
11                 TreeNode temp = q.poll();
12                 arr.add(temp.val);
13                 if(temp.left != null) q.offer(temp.left);
14                 if(temp.right != null) q.offer(temp.right);
15             }
16             result.add(arr);
17         }
18         return result;
19     }