BinryTree Traversal preorder return preorder Expression as String

[javasuniltyata]

Hi,

I have problem with following method that we used for BinryTree Traversal. The follow method is good so far.

public void preorderHelper(MyBinaryTreeNode rt){
 if(rt==null)
 return;
 System.out.print(" "+rt.data);
 preorderHelper(rt.left);
 preorderHelper(rt.right); 
}

But,


Now I have to change this method in such a way that it will return String(which will contains preorder expression).

public String preorderHelper(MyBinaryTreeNode rt){
  /*

 ??????????
 */
}

Any Idea. Thank you in Advance.