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   }