Return a function that returns the second largest item in a binary search
tree (i.e. BST).
A BST is a tree where each node has no more than two children (i.e. one left
child and one right child). All of the values in a BST's left subtree must be
less than the value of the root node; all of the values in a BST's right subtree
must be greater than the value of the root node; both left and right subtrees
must also be BSTs themselves.
I solved this problem thrice -- improving the performance profile each time. The
final solution has a runtime complexity of O(n) and a spacetime complexity of
O(1).
