Since a leftist heap is a heap-ordered binary tree,
it inherits from the `BinaryTree` base class
the three member variables: `key`, `left` and `right`.
The `key` is a pointer to the object contained in the given node
and the `left` and `right` member variables point to the
left and right subtrees of the given node, respectively.
In addition, the member variable `nullPathLength` records
the null path length of the given node.
By recording the null path length in the node,
it is possible to check the leftist heap balance condition in constant time.

Copyright © 1997 by Bruno R. Preiss, P.Eng. All rights reserved.