add() function in linked-list.js

[gabivlj]

In the add() function in linked-list.js I see the following line of code:
if (current.next) { current.next.previous = newNode; } // <7>
What is the reason behind this line of code? If we are inserting behind current, I don't know why would you set the current.next.previous to the node we wanna insert.

https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L83-L105

[amejiarosario]

The short answer is... You need to update current.next.previous because it needs to point to the newly created node. When you add a new element to the middle of the list you need to update the 4 references around it and this is one of them.

---

The long answer is...

I'll use the example on the docs to explain this line better.

Let's say we have the following linked list:

art -> dog -> cat

and we want to insert a node called "new" on the position 1 (middle of the list).

https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L92

So, current would be dog node, since is the one currently on position 1.

After we create the "new" element, to add it to the middle of the list we need to update 4 links

1. new.previous
2. new.next
3. art.next
4. dog.previous

image

In code, it looks like this:
https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L92-L99

So after we create the node, we need to update 4 references:

1. Update node link new.previous: https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L95

2. Update node link new.next: https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L96

3. Update node link art.next: https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L98

4. Update node link dog.previous: https://github.com/amejiarosario/dsa.js/blob/cc1ccc0183ea5d4675815413b7d1d4cc14da348c/src/data-structures/linked-lists/linked-list.js#L99

[amejiarosario]

@gabivlj Let me know if that explanation makes sense or if you still have doubts. I might add it to the main docs if this helps to understand.

[gabivlj]

Shouldn’t it be current.previous = newNode instead of current.next.previous = newNode?

[amejiarosario]

@gabivlj You are totally right! It should be current.previous = newNode. I'll update the code/docs/tests. Thanks for reporting this!