In java:

Write a method named sum that returns the sum of the data values stored in a binary tree. Your method accepts as its parameter a TreeNode that refers to the root of the tree. For example, the following tree has a sum of 33, so if a variable named tree refers to its root, the call of sum(tree) should return 33.

 _____________3 / \ -9____ 15 / \ \ 12 24 0 / \ 3 -9 / \ 0 -6

Assume that you are interacting with TreeNodes as defined below:

public class TreeNode {

 public int data;

 public TreeNode left;

 public TreeNode right;

 public TreeNode() { ... }

 public TreeNode(int data) { ... }

 public TreeNode(int data, TreeNode left, TreeNode right) { ... }

}