Finding the kth element in Binary search tree

Given a binary search tree, how do we find a kth smallest or kth largest element?
For example, given the following binary search tree.
Third largest element is 6
Second smallest element is 2
Fifth largest element is 5 and so on…


We can solve this problem by modifying the the in-order traversal method of a binary tree. In addition to the root node, we can pass two more parameters one is K, and current count of the nodes visited as a reference parameter. When the current count reaches K we found the kth order element.

To find out the kth smallest element, we need to visit left sub-tree, then root and then the right sub-tree as usual. To find the kth largest element we need to do reverse in-order traversal i.e First visit right sub-tree, then root and then the left sub-tree.

Here is the C++ implementation. This includes the recursive and iteration versions of finding kth smallest and kth largest elements. The iterative version is simply the manual implementation of a recursion using a stack.