Fix/splaytree delete last node

[B7M]

Fix crash when deleting the last remaining node in a Splay Tree

---------- PR DESCRIPTION BELOW THIS LINE ----------

Problem

In SplayTree.treeDelete, the root is replaced by its left child and then written to right away:

this.treeRoot = this.treeRoot.left;
this.treeRoot.parent = null;

When there is only one node in the tree, that node has no left child and no right child. The code takes this branch because the right child is null, so this.treeRoot becomes null too. The next line then tries to set parent on null and throws:

TypeError: Cannot set properties of null (setting 'parent')

The error also stops the animation commands for that delete from running, and one of those commands is the one that erases the node's circle. So the node the user just deleted is still drawn on the screen, even though the tree is now empty. The two stay out of sync until you press Clear.

This is not only the one-node case. Any sequence that empties the tree hits it, because the last delete always happens when a single node is left.

Fix

I checked for null before setting parent:

 this.treeRoot = this.treeRoot.left;
-this.treeRoot.parent = null;
+if (this.treeRoot != null) {
+	this.treeRoot.parent = null;
+}
 this.resizeTree();

The tree is allowed to be empty here, so a null root is a normal result and not an error. resizeTree() already checks for a null root, so nothing else needed to change.

I did not change the other branch (left == null). That one only runs when there is a right child, so its new root can never be null.

Testing

I tested the change in the browser and looked at what was actually drawn on the canvas at the end. nodes below is what stays on the screen.

Steps	Before	After
insert 5, delete 5	error, nodes=[5] ❌	no error, nodes=[] ✅
insert 5, delete 5, insert 7	error, nodes=[5,7] ❌	no error, nodes=[7] ✅
insert 5, delete 5, insert 7, delete 7	2 errors, nodes=[5,7] ❌	no error, nodes=[] ✅
insert 5,3, delete 3, delete 5	error, nodes=[5] ❌	no error, nodes=[] ✅
insert 5,3,8, delete 3,5,8	error, nodes=[8] ❌	no error, nodes=[] ✅
insert 5, delete 5, insert 5,2,9	error, nodes=[5,5,2,9] ❌	no error, nodes=[5,2,9] ✅

The last row shows the leftover node most clearly: the old 5 is still there, so the canvas draws 5 twice.

I also checked the cases that were already working, to make sure the fix did not change them. These behave the same before and after:

Steps	Result
insert 5,3, delete 5 (root with one child)	nodes=[3], no error
insert 5,3,8, delete 5 (root with two children)	nodes=[3,8], no error
insert 5, delete 9 (value not in the tree)	nodes=[5], no error
delete 9 from an empty tree	nodes=[], no error

To see the bug you have to delete the tree down to empty by hand, which is probably why it went unnoticed — the Random button only inserts, it never deletes.

Checks

• npm run lint passed
• Prettier check passed for the changed file
• Only src/algo/SplayTree.js was changed (3 lines added, 1 removed)

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