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

  class TreeNode {
       int val;
       TreeNode left;
       TreeNode right;
       TreeNode(int x) { val = x; }

       @Override
       public String toString(){
           if(this == null) return "";
           StringBuilder sb = new StringBuilder();
           LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
           queue.add(this);
           sb.append(val + ",");
           while(!queue.isEmpty()){
               TreeNode tmp = queue.poll();
               sb.append((tmp.left == null ? "#" : tmp.left.val) + ",");
               sb.append((tmp.right == null ? "#" : tmp.right.val) + ",");
               if(tmp.left != null) queue.add(tmp.left);
               if(tmp.right != null) queue.add(tmp.right);
           }
         return sb.toString();

       }
   }