$20.$ Saddle Point:

Input: A binary tree T in which each node x contains a field value $\backslash ([$x$]\backslash )$ which is a real number.

We say a node x in T is a saddle point if x has minimum value among all its ancestors $($including x $),$ but it has maximum value among all its descendants $($including x $).$

Output: A saddle point of T if there is one, or null if there isn't.