Data Structures: Stacks and Queues

[@enjoyinglife9853]

You explained this better in 5 mins, than my professor could in 2 hours! Thank you so much!

[@abdulmohibabdulrahman6924]

Legend 🙄🙄

[@argonred]

So very true

[@mutatednut]

Better than my professor could in 3 weeks

[@drzlilj]

Better than my professor could in 2 semesters

[@tannerbarcelos6880]

It is funny when you get a pretty good foundation in your intro to comp sci courses how easily it is to understand these data structures. Then again, Gayle is an amazing teacher! Not a luxury we all get with college, lol.

[@tan8067]

Thank you very much ma'am. Stacks and Queue's have been stressing me out for the longest, but now I have actually taken the time to sit down and understand them and they are actually fairly simple, at the mention of pointers, I used to tremble, but now I realise that I have got nothing to fear at all!

[@Digger-Nick]

7 months late, but a stack and a queue are the most basic concepts to understand...

[@tan8067]

@@Digger-Nick I agree. On paper, they're VERY easy to understand. I just had trouble implementing them when it came to actual problems.

[@hilapartuk6769]

Greate video! One suggestion - When implementing a Stack, it makes more sense to call the Node's pointer "prev" than "next", because logically it doesn't point to the one that comes after him :)

[@laurafigueroa9922]

true, I called mine 'down' :)

[@vaibhavyadav-patil3695]

'down' makes much more sense.

[@laurafigueroa9922]

@@vaibhavyadav-patil3695 yea! Tho I was told it wasn't best practice 🤦‍♀️

[@vaibhavyadav-patil3695]

@@laurafigueroa9922 why & who told you that?

[@hilapartuk6769]

@@vaibhavyadav-patil3695 you're right, but Gayle used the word prev in her other videos (I think it was LinkedList, not sure)... so just to be consistent with her other tutorials :)

[@najeebiBrahvi]

Very Very simply explained. It is better than the Course which I bought from Apna College!

[@martinmtandi335]

awesome vid. you are the first person who has really explained the coding of this algorithm in a manner i actually understood.

[@dragonore2009]

At the university you do as the video is depicting. At least from C++ a person can just easily use the built Queue and Stack library, but you learn nothing. At the university they show you how a Stack and Queue work, like the video and then you implement it. You are not allowed to use any STL libraries. I learned alot, when I was restricted from using STLs and I would recommend everyone make a program that can easily be done with STL libraries, but to not use them, this forces you to understand programming better.

[@dragonore2009]

Obviously I don't mean production code, but for someone who is learning.

[@johnsena0007]

You're really have a point!

[@geekyprogrammer4831]

@@dragonore2009 you are right!

[@vela7447]

Huh, at my uni we used nodes for linked lists and trees, but python lists for stacks/queues... This was a far better explanation.

[@xynyde0]

@@vela7447 python lists implementations are similar to dynamic array implimentations. Where as in this video stacks and queues are the implementations of linked lists.

[@CSEconceptswithParinita]

Thank you so much ma'am! You're an inspiration for new tutors like me🙏💙💙💙

[@silvershocknicktail6638]

I see so many tutorials for coding using this same doodling/animation software, but I don't recognise it. What is it?

[@procrastinator0357]

Although I understand what stack is and how to implement, I’m struggling to see when I can use stack, like in which “actual programming” situations.

[@kulsumkhansla4101]

I think we can use in when we want to check some kind of grammar or syntax, usually in the case of a compiler( lexer and parser)

[@amadmlinka4812]

It is also useful for implementing a post-fix notation system, if you want some user inputed data to be calculated.

[@fahimhassan9024]

This is my question too lol. I understand the implementation I guess, but I have no idea where this might be required :3

[@Digger-Nick]

@@fahimhassan9024 It's not as important or difficult as these people make it out to be.

[@Edsiaren]

I had to use it on a calculator app that converted infix (1+2*3) to postfix(2 3 * 1 +) before calculating the problem. Converting to postfix helped a lot with nested problems (i.e. 1+2(3*4(1(3-2)))-4*3(1)) for me. I'm sure there's a work around without having to convert it to postfix, but finding a way to dig inside that parentheses and making sure those problems are solved in order was pretty troublesome. Look up postfix conversion or "Reverse Polish Notation" (RPN) on youtube, one of the videos show how a stack is used to determine what problems are going to get solved first.

[@frankchen3021]

Your videos are always so straight-foward!

[@YoniFraimorice]

Node class in 4:20 has a private constructor ,why's that? the class Node is private, that's fine since we're only using it and exposing it to our solution class, however creating the c'tor of the Node class as private won't allow us to instantiate it (only from within the Node class)

[@martinmtandi335]

got the same qstn too

[@pascal3889]

The outer class can access the private methods and fields of the inner class.

[@gurukarthi]

Since, We are supposed to create object for the class Stack/Queue not for the class Node. Because the class Stack/Queue will take care of creating the object for the class Node. Moreover, The class Node is created to make use of data organization inside the Stack/Queue not from outside!..peace!

[@mohdbilal6561]

very well understood.Thanks mam from India 🇮🇳

[@luiscarlosquesadasequeira1548]

Can anyone explain at 3:33, in the remove method, why the tail need to be set to null after updating/removing the head? L36.

[@CaveDave]

This is very confusing, it would have been much more simple to follow along if you would have kept your "line of people" analogy from the beginning 1:00 where the "tail" is on the left side making more sense because new people (nodes) will enqueue on the left side to represent an growing queue. Now you have mixed it all up: 2:40 . You add a new node but put it on the right side and make it part of the tail.

[@rumelia]

This series of Data Structures is great! Succinct, clear, and to the point. I only wished they used C++ since I got a final of it coming up :D

[@englotv]

this is true coding. you helped me understanding sta cks

[@danielabengona5142]

Where do you think it would be applicable to use them in some certain programs?

[@kataya5005]

does it make sense that node is a static class, since we're making instances of the node with different data ?

[@mohamedalifrani5416]

Interesting implementation. But "top-level static classes" don't exist in Java! So the Queue class must just be a public class. In other cases if you need a "top-level static class" you can declare it as "public final", make its constructor "private" and its members as "static".

[@somerandomguy000]

that happens when you’re just reading your teleprompter without actually knowing the thing

[@deli5777]

Thanks for this, you made the code seem so much simpler

[@theSeniorSDE]

Thanks for these easy videos to undertand.

[@Eric-xh9ee]

This is by far the best explanation I've seen! PS why didn't you delete the node you popped, in the stack? I thought you always delete nodes you add otherwise you'll run out of memory

[@DopetheKid]

because technically it is deleted. nothing is pointing to it anymore so its garbage collected.. when using an array, that is when you should delete because array takes up a block of memory at a time.

[@凛-r3h]

Stacks and queue are pretty easy to remember their meaning. Its the same as the english meaning of stacking and queue!

[@robertbonham6621]

Currently working on a class project where we can choose to either do stack or queue, which one would be better implement/ use. Stacks or Queues?

[@ngndnd]

for the pop method in stack, it is removing the element that i added first, not the one that i added last. Or maybe im just getting confused bc of the way its actually implemented in code, idk

[@hasansakib3502]

Sometimes i wonder how many subscribers did you get after these wonderful lectures??

[@loummad9552]

I dont understand this line plz (@3:37) if head == null ===> tail == null;

[@denver832]

Is 2:33 's code executable i think its not.

[@joyalmeida6565]

can you suggest me a title for a project which uses data structure in c. I need a project which uses link list/ stack / queues

[@piero9665]

Good explanation!!! thanks Gayle

[@dennisrufi143]

Hello, Very well explained but I am confused with remove method. How is it actually removing a particular node in Queue?

[@sonnix31]

Lol, well explained, but you are confused? Watch Durga Sir and you'll understand.

[@growingprogrammer6593]

She actually just removes any reference to it. This means that the Garbage Collector, speaking in terms of JavaEnv, will delete it because it is a data without references.

[@xynyde0]

in garbage collected languages like java and python, when an object has no references or is 'unreachable', it is deleted by the garbage collector. But if you are writing the same data structures in languages like C++, you will have to manage memory by yourself. Which means, you will allocate and deallocate memory for any objects you create.

[@jamesyoo67]

Question for Stack.Pop(). Why do we not destroy the old Top object and set it to null, after setting the new Top?

[@jamesyoo67]

Edit: looking at StackOverflow, in Java and C# it is worth it to null out the reference after popping it.

[@BrandonAB]

When the camp counselor tells everyone to line up at the flag pole and then when everyone does, he says to turn around, that’s LCFS.

[@shibinfrancis9826]

How did you add first tail? If(head == null){ head = node; tail ?}

[@anjani0agrawal]

For Queue, add. If head is null shouldn't head and tail both be assigned to Node?

[@DONofC0D]

That is what she's done there. In the add method she will always set the tail to the node. And if head is null, she has also set head to the node. This means head and tail points to the same node

[@feifeiliu276]

Thanks! So easy to understand.

[@sakshamp4488]

Nice explanation. 🤩

[@Kuriocity]

*I dont understand the tail node object why you are overwriting it in the add method???*

[@pablocs12]

Is not overwriting it, at that point of the code (line 24) the "tail" object is still an instance of to the penultimate node (the 3th in the graph), what is doing there is switching the reference of its next node to the new tail. So the "next" node of 3th one ,is now the 4th node. I hope I've made myself understood.

[@abtest5053]

Is it necessary to put Node class inside stack class?

[@uyarahmet]

Sound of the keys, OMG :D

[@sundarb6673]

Thank you, Gayle!

[@DopetheKid]

i dont really understand the if(head == null){ tail == null; } line in remove() of the queue. can someone explain?

[@mustafak9565]

in the add method of quene, cant we just let tail.next= node without the if(tail!=null) check?

[@scpancakes]

It really depends on your compiler and a number of other factors, but when you try to access data members from a null pointer you will experience "undefined" behavior. In other words, you may be trying to access data from an object that does not exist! In my case it always ends in a segmentation fault. It's always best to avoid code that results in unexpected behavior. Note: coming from c++ but I imagine its much the same

[@XxGummib3arxX]

Yes, you can do something like this public void add(char c) { if (head==null) { Node newNode=new Node(c); head=newNode; tail=newNode; } else { Node newNode=new Node(c); tail.next=newNode; tail=newNode; } }

[@abdulrashid1060]

At 3:17 you said "Remove from head of linked list" :)

[@thewatcherlollol]

Well, most of these Data Structures are implemented using Linked Lists.

[@baljeetsingh5813]

which ide you were using?

[@danielabengona5142]

Why do you think it is appropriate in some certain programs?

[@aishikbhattacharya4296]

Why cant we use a different class for Node as we did for linkedlist

[@Beziu1]

why remove method returns data instead of void ?

[@Lea7herFace]

it should be called dequeue

[@TheMediocreDev]

It's actually changing the data (setting the tail to null) instead of just manipulating the pointers. Basically, need to return method with .data but not just .next()

[@_danzo7]

because its have to return the value that had been removed like when u r in a real queue and u r an integer when its ur turn they cant just say next and ignore u

[@mukulsaini1659]

what would be the time complexity of this?

[@williamplay1000]

Why are the nodes static?

[@kyy6150]

why do you need "Next" and "Tail" reference variables, why cant you just use "Next" as the tail instead of having 2 reference variable referring to the same object.

[@MinecraftLetstime]

Then you have to traverse the queue to insert an element to the tail.

[@growingprogrammer6593]

This way you can avoid going through the queue to insert an element (because you already have the tail!)

[@hmak5423]

Amazing. Just amazing. Btw, I clicked because I thought Charlize Theron is the teacher.

[@cenkercanbulut3069]

Well explained, Subbed!.

[@moviefanie]

Nothing wrong with this video. I found in some interviews, for Queue people especially look for enqueue and dequeue methods instead of remove and add.

[@thadtayo]

Why does she create a Stack class and Queue class? Are these data structures not built into Java?

[@revdevstudios]

they are but she teaches you the implementation and how these data structures work.

[@XxGummib3arxX]

They are are in the Java api, but the point of the video is to show you how they could possibly be implemented and how it works internally. In her case using a linked list. What if you went to a job interview and they asked you to implement a queue? You would probably fail in that case.

[@joecamroberon9322]

Because the video would be 30 seconds long is she did that.

[@736939]

I though that it's null when head==tail on 2:13

[@zentes08]

Not necessary. If we have 1 element in the queue, head==tail is true.

[@yoitslemonboy6988]

can we please get the source?

[@syedmuqeet]

I got confused with stack push pop operation

[@mergatafa8501]

excellent lecture

[@shoban2090]

Why do we keep 'Node' as a inner class?

[@officialsterlingarcher]

so that it can't be accessed outside of the queue or stack class. It doesn't make much sense on why we don't want to access it outside of this class in this scenario. But say that you're building an algorithms library for other programmers to use and you add these data structures to it. You don't want those other programmers to accidentally access the node class and start messing around with raw data without having to go through the helper functions in the outer class. That would defeat the whole purpose of the data structure you're making.

[@jamesyoo67]

@@officialsterlingarcher In her sample code, I can't manipulate her Stack's data because A) Top is a private member of Stack, and B) data is a private member of Node, both with no public setters or getters. Defining a private Node class within a Queue class means, outside the Queue class, I cannot freely create and use Node objects on my own.

[@ms.r1611]

Wow I didn't know the face behind the book: Cracking the coding interview.

[@greerhanshaw]

At the risk of being tedious, can someone explain: is a stack/queue a data structure (as described here), an abstract data type (as described on Wikipedia) or both?

[@GuardianTam]

It's both. it's an abstract data type when you're talking about how it behaves. it becomes a data structure when you actually implement it, since there are many different ways to do so.

[@jacksonjoestar5035]

A stack can be an abstract data structure, remember that an abstract data structure is A class where not all methods are initialized.

[@nafeesahmed4942]

How i can use this editor for practice ?

[@anjaligoyal1754]

hackerrank.com

[@victor3btn598]

Why is stack like a linked list

[@hao5868]

Which IDE was used in this tutorial? Thanks!

[@growingprogrammer6593]

It is the default code section used in HackerRank

[@Ibrah_id]

Very clear and informative ❤️

[@jamesperez6964]

What is the deal of the dot operator being used with the 'this' pointer? this.data = data; instead of, this->data = data; ???

[@HafisCZ]

Because it's Java, not C++. In Java operator - > is for lambda. In C++ 'a.x' returns member x of the object a, and 'a->x' is short for (*a) .b (returning member of object's pointer). It's explained there better: stackoverflow.com/questions/4984600/when-do-i-use-a-dot-arrow-or-double-colon-to-refer-to-members-of-a-class-in-c

[@averagehooligan620]

So helpful! thx

[@NinjaTuna9]

A stack is just a singly-linked list??? WTFFFF

[@jx_cob5988]

True, it really just defines how items are removed and added to it. There is some other stuff if you go deeper however.

[@danielbreen3945]

_intentionally keeps her own book at the bottom of her "stack" to show humility_

[@M4th3u54ndr4d3]

@@charlesj9106 lol

[@Jemmeh]

Thank you for the video. I suggest using a pop filter with the microphone next time though, there's a lot of little pops in the audio and it's distracting.

[@MosesHashim3]

the idea of stack and queue is simple, but everyone has a different way to code it. I could not follow you and read what you were coding.

[@Tux0xFF]

a static class that contains another inner class, i need to re learn java

[@whitebird365]

Thank you!

[@jl-dq5ch]

waiting an hour or two for a movie?!

[@collegestudent6071]

Why don't these tutorials specify the language...

[@charleskenney3574]

Wow thats good schema

[@benzeltser9851]

why did it take my teacher 3 hours to teach at, and not as good as this video tbh.

[@tedyhy24]

It's great!

[@AshokAshok-ig9jk]

Super

[@brd8764]

FILO and LIFO.

[@skylarkim22]

thanks!

[@0xssff]

but why Java (eww) tho?

[@fatimaaw]

thank u!

[@RogerKeulen]

Wrong, Fake news: The _head_ is the first element. The rest is the _tail_. ;-)

[@adamsadam366]

nice! =)

[@duncanphillipgreen]

Hm, I don't think this will even compile!

[@KwabenaAning]

Yeah...

[@rasam.priyan]

I feel sad ... Just because she used Java to teach stacks and queues...

[@sim1manson1]

FAKE QUEUES!!!

[@fernandoluis53]

You did not talk about big-o at all. very unprofessional

[@KelleyKouture]

she has a whole video on big-o. This video was about stacks/queues and it was very well done