override toString() function for TreeNode to output OJ's Binary Tree Serialization

 1  class TreeNode {
 2       int val;
 3       TreeNode left;
 4       TreeNode right;
 5       TreeNode(int x) { val = x; }
 6       
 7       @Override
 8       public String toString(){
 9           if(this == null) return "";
10           StringBuilder sb = new StringBuilder();
11           LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
12           queue.add(this);
13           sb.append(val + ",");
14           while(!queue.isEmpty()){
15               TreeNode tmp = queue.poll();
16               sb.append((tmp.left == null ? "#" : tmp.left.val) + ",");
17               sb.append((tmp.right == null ? "#" : tmp.right.val) + ",");
18               if(tmp.left != null) queue.add(tmp.left);
19               if(tmp.right != null) queue.add(tmp.right);
20           }
21         return sb.toString();
22           
23       }
24   }