Number of nil-links in a binary tree
The Wikipedia article defines a null links as "absent children of the nodes", presumably "absent" in the sense of what nodes are missing to obtain a perfect binary tree (i.e., "a binary tree in which all interior nodes have two children and all leaves have the same depth or same level"). Under this definition, leaves (i.e., the nodes in the last level of the tree) do not have links. Hence, because all other levels are filled, to determine the number of null links you need only look at the previous to last level of the tree. This also explains the maximum case (i.e., "where only 1 node exists in bottom-most level")