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();
          
      }
  }