Whenever I come back to implementing the most basic method for singly linked lists (push
), I'm always scratching my head, thinking to myself, "how did I implement that again?". So here I am once again my friends, typing out concepts I don't fully understand in order to make sense of them.
Linked lists at their core are simply alternatives to using a basic array. Why might we choose a linked list over an array, you ask? Let's go over the benefits:
1.) More efficient insertion/deletion processes (typically an O(1) time complexity) over standard arrays
2.) Useful for datasets where the exact number of items isn't known; this makes for a much more flexible data structure that can be changed more efficiently
3.) Serialization (conversion from an object into bytes for transfer) is more efficient with linked lists than arrays
As with all data structures however, there are downsides to using them and linked lists are no exception. When might we NOT want to use a linked list? Let's go over the drawbacks:
1.) If your dataset requires a lot of data inquiries (look-ups), linked lists will perform worse than arrays. This is due to the fact that their location in memory (e.g., an index position) doesn't typically exist (but it can; the efficiency of that look-up will still perform worse than a basic array though)
2.) Smaller datasets won't necessarily benefit from any of the reasons for using a linked list. It may be more useful to simply use an array if the cost/benefit in time and space complexity for insertions/deletions won't be a cause for concern
So in summary for its use-cases:
- Linked lists are good for when you have a large dataset and it will mainly be used for insertion/deletion of data
And for its drawbacks:
- It probably won't be of much use if you have a small dataset that will mainly be used for searches and data-manipulation
Alrighty, let's dive into implementing the basic structure of a linked list and its most basic push
method:
We first have our two constructor functions that will be the backbone in building our linked list; the LL
will be where we have our head
, tail
and length
properties.
The head
is our entry point into the linked list. All data will be passed through the head in order to populate our list.
The tail
is our terminal point for the linked list. The tail will always contain the very last data point in our list. We don't necessarily need to have a tail in a singly linked list, but it does make it more efficient with finding the very end of the list.
The length
property is simply what it sounds like. It'll let us know how many items are contained within the list.
As for the Node
constructor, its sole purpose is to add onto the linked list. It will contain some data (val
) and a next
pointer that will be linked to the subsequent node in the list. The default value for next
will be null to accommodate any future node insertions.
Now let's explore our first method:
PUSH
If you're seeing LL.prototype.push
and are thinking "the heck is a prototype?", this is a great resource. It's a bit lengthy, but I can almost guarantee it will cement the concept for you.
And if you're wondering why I'm not using ES6+ class syntax, it's because I'm exercising my constructor/prototype muscles, as they're getting flabby.
ANYWHO
What we're trying to accomplish with the push
method is very similar to the built-in array method by the same name in JavaScript; we simply want to add some data to the end of our list. This is first accomplished by assigning the desired piece of data (or node) to a new instance of our Node
constructor we defined previously:
After that, the first thing we want to check for is if we even have anything assigned to our head
property, which as you remember, is the entry point into our linked list. If we don't have anything assigned to head
, then we'll want to populate it first, as well as assigning that same piece of data to our tail
property:
The reason we want to assign this.tail
to this.head
is because any new piece of data (newNode
) that we are inserting into the end of our list will need to mirror our tail. This will become more apparent once we dive further into this method. Bare with me, I know it's confusing as heck the first few (or dozen+) times you're trying to make sense of this.
So now that we have our node to insert and we've checked our head
to see if it's empty, let's see what we do once our head
is actually occupied:
This is a duplicate assignment we're doing for two different properties. this.tail.next
is being populated by the Node
constructor with what we've assigned to our constant variable newNode
.
The reason our LL
and Node
constructors are linked is thanks to the previous line of code we performed:
this.tail.next
is essentially just this.head.next
Read that again.
And then again.
And then look at the code and then read it again.
⁽ˢᵉʳᶦᵒᵘˢˡʸ ᵖˡᵉᵃˢᵉ ᵈᵒ ᶦᵗ⁾
That previous point is essentially the crux of how this is all working. We have our Node
constructor which will be assigned to the this.head
and this.tail
properties in our LL
constructor and all future data will be passed through both of those properties once our initial head
is populated.
Once we've arrived to our third push
invocation of data insertion, we'll only be working from the tail from that point on. The head exists to initially populate our list and then act as a liaison to transfer its population onto the proceeding tail properties.
Again, to reiterate, once we've populated this.head
and assigned this.tail
to this.head
, we're simply populating the tail and the inherent next
property that the tail has with any subsequent data.
If that's not fully making sense, don't worry. Even upon taking my time and typing all of this out, I'm having to second-guess what I'm typing out. This are very unintuitive concepts.
BUT if you were able to make sense of that, congratulations! Either you knew about that already or I, by some miraculous intervention, clarified that a little for you. All that's left to do is return out from the method and increment the this.length
property on the LL
constructor to signify that we've populated our list:
Some implementations of the push
method have multiple sections where they increase the this.length
property, but I like this rendition as it adheres to the fabled DRY principle.
And that's all I have to give, friends. You can bet your bottom dollar I'll be back with the 𝓈𝓅𝑜𝑜𝓀𝓎 pop
method for linked lists when I get around to it.
~ Thanks for stoppin' by! ~
<3<3
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