Java: Find next right node of a given key

Problem Statement:

Given a Binary tree and a key in the binary tree, find the node right to the given key. If there is

no node on right side, then return NULL. Expected time complexity is O(n) where n is the number

of nodes in the given binary tree.

Example

Consider the following Binary Tree. Output for 2 is 6, output for 4 is 5. Output for 10, 6 and 5 is

NULL.

 10
 / \
 2  6
/ \  \
8   4  5