Example: 4 / \ 2 8 / \ 5 10 Output ==> 10
1. If the root node is null, return null.
root
null
2. If the right node of root is null, return root.
3. Else recurse through right nodes. i.e. return findMax(root.right)
return findMax(root.right)
public TreeNode findMax(TreeNode root) { }
