class TreeNode {
String val;
TreeNode left;
TreeNode right;
}

implement two API:

String serialize(TreeNode root)
TreeNode deserialize(String code)

Pay attention to these cases:
node = null;
node.val = null;
node.val = ""; (empty string)
node.val = ",# null4\n" (can be any unicode string with empty space etc.)

example:

				",\#  4"
			/             \
		 (null)        "null"	
         /                /         \
	  ""	          "2a"     "/.,%\n"  
	
	

(null) above represents node != null && node.val == null