A leftist heap is a node-based data structure where push, pop and merging of two heaps
may all be performed in O(ln(*n*)) time. This depends on a property called the
minimum null-path length. A null path is any path from the root of a binary tree to a node that
does not have two children. The minimum null-path length is the minimum length of all null paths
within a tree. If the minimum null-path length of an empty node is defined as -1, we may recursive
define the minimum null-path length of a tree with a non-null root as being the mininum of one plus
the minimum null-path length of the left subtree and one plus the minimum null-path length of
the right subtree.

A leftist heap may be defined recursively:

- A leaf node is a leftist heap,
- A non-leaf node is a leftist heap if both subtrees are leftist heaps and the
minimum null-path length of the left subtree is greater or equal to the
minimum null-path length of the right subtree.

The primary operation of leftist heaps is a merge operation of two leftist heaps. Once
this is defined, we may define push and pop in terms of merge operations:

- Pushing a single object onto a leftist heap is the same as merging the
exisiting leftist heap with a leftist heap comprised of a single node, and
- Popping the minimum element from a leftist heap is equivalent to removing
the root node and then merging the right subtree with the left subtree.

## The Merge Operation