Find Kth node in binary tree.

/**
 * FindKthNodeInBinaryTree.java
 * 
 * Description: You have a binary tree where each node knows the number
of nodes
 * in its sub-tree (including itself). Given a node n and an int k,
write a
 * function to return the kth node in an in order traversal.
 * 
 * @author John Qiao <qiaoshun8888@gmail.com> Date: Jan 25, 2014
 */