为什么给定的二叉树用空节点构造？

Question

I have started to read about the trees structures and binary tree comes the first.

I use the following resource: https://runestone.academy/runestone/static/pythonds/Trees/ListofListsRepresentation.html

There you can find the binary tree image:

With the list of list representation it looks like:

And my questions are : why do we need the empty node marked with arrow, if it is not displayed on the binary tree image? As we know the binary tree can have up to two nodes, so, why do we need this empty node? How does this empty node help us? Or it's just the convention which we should use?

Answer 1

This is mostly explained in the link, specifically with the constructor function:

def BinaryTree(r):
  return [r, [], []]

When a new leaf is created, it always contains:

• The value itself
• The left node
• The right node

This has the advantage that when you parse the tree, you do not have to check if left and right exist, as they always exist. This makes the code more regular and simple.