Data Structures: Heaps

Learn about heaps. This video is a part of HackerRank's Cracking The Coding Interview Tutorial with Gayle Laakmann McDowell. www.hackerrank....

[@Saztog1425]

3:22 "Aaand there we go, we've created Minecraft!"

[@AlanD20]

EXACTLYYYY 😂😂😂

[@lilypad5182]

For heapingDown, what if instead of the left child being swapped, the right child was being swapped, and the new node would get bubbled down to a place that exceeds the size? then the heap will no longer be compact and there would be empty spots, no? So we'd need additional implementations to take care of this case

[@owinophil5777]

why are we swapping indices instead of elements in the heapifyUp method?

[@stevenmarsh4051]

What software is she using to show the handwritten aids?

[@martinduva1339]

Excelent video! Helped me so much, thank you

[@sherazdotnet]

Just an FYI: At 3:01 timeframe, you are showing formulas and for pareint you have [Index -2) / 2]. This needs to be change dto index -1 * 2. On next screen where you are coding it, you have it right.

[@Pazushh]

I think it shouldv'e been (Index-1)/2. while "/" rounds to bottom

[@calviethang7139]

You are right!

[@SupunNimantha]

Actually that equation is not for the immediate parent of any given node but it gives you the min node (top most node ). Instead of saying parent she should have told its the min. There she made a mistake. At the same time actually there is no need to have that equation because simply the 0th element is always the min.

[@dennisfolz352]

@@SupunNimantha no you are wrong. The formula (index-1)/2 returns the parent for any given node. And it is important, because you need the parent of any given node if you want to heapify up. ^^

[@santjagocorkez]

@@SupunNimantha You could do the maths yourself: take the 9 at #3. Its parent is 4 at #1. Now let's compute: (3 - 2) / 2 = 0 (floor div). Oopsie, 9 at #3 has the root as its parent, while we know from the picture it's not.

[@hammerain93]

If you're trying to write this code in Python, beware: You cannot simply assign items[0] = items[self.size - 1]. You must pop() the item at the end of the list to remove it: items[0] = items.pop() ... also be sure to use floor division in the parent calc if using Python 3: (index - 1) // 2

[@leonardom.deoliveira4465]

Why not though?

[@ggd123]

@@leonardom.deoliveira4465 The item at the end of the list still exists after you reassign items[0] so essentially you have a duplicate of the last item at the front.

[@PsyKosh]

Possible error around 2:52 The diagram shows the parent as (index-2)/2, when it should be (index-1)/2

[@g.o.4678]

I believe that calculation takes the ceiling (or whole integer value rounded up, depending on the programming language) of the operation. So, for instance, to get the parent of the node at index 7, we'd have: ceiling((7-2)/2) = ceiling(5/2) = ceiling(2.5) = 3, which is the appropriate index we're looking for.

[@arvinsim]

Gbenga Ojo

[@DimaKurguzov]

Your are right. (index-2)/2 for parent index is a mistake. Look the code at 3:22 - here is the correct version (index-1)/2.

[@quangthang10d4]

Yes I was gonna say the same thing!

[@josevillegas2721]

In python 2, / is integer division and it truncates the result so 5/2 = truncate(2.5) = 2

[@harshitm6403]

Storing the heap in the form of an array just blew my mind...so effective

[@theinverteddonkey2961]

it's really a tree in the form of a list of nodes

[@typingcat]

Damn, if that blows your mind, your mind most be blown multiple times a day.

[@davidlee588]

@@typingcat haha, i'm also been wondering why people easily got blown away by simply KZbin videos, it must be like an ejaculation moment for them. 😂

[@vectoralphaSec]

@@typingcat mine is. There is so much to learn every day. My mind is blown on a daily basis. Its great because im never bored.

[@hamzahkhan878]

what in the hell we were you thinking of if that blew your mind? lol

[@m2rafik]

Most of coders strugles to use complex abstract data structures like heaps or graphs because they dont learn it from a concrete use case.

[@sarvasvarora]

+1 for this. Doing an intro to CS course at uni rn and def if it wasn't for the coding assignments involving practical usr cases, I would've never appreciated such data structures... It's certainly very important to actually implement them in some use case in order to grasp them well.

[@stas4985]

why the hell graphs or heaps complex???

[@axea4554]

@@stas4985 because they are more complex than a simple non-resizable array or a linked list

[@harshwardhankoushik8515]

The explanation with the code is amazzing !! loved it....seems that would work for me! Please continue with the good work

[@johnkimura5585]

Her keyboard clicks are the most satisfying sound

[@RobsRobotChannel]

ASMR, wikipedia it.

[@ivanleon6164]

hate it.

[@himanshusingh5118]

😂😂😂 irritating

[@sanjayvasnani988]

Nah. It seems as if her keyboard hates being used by her.

[@NilakshMalpotra]

Agreed! Lovely sound

[@NathanSowatskey]

The calculation shown in the cartoon diagram to get the parent of the node is shown as (index-2)/2. In the code the calculation is (index-1)/2.

[@RachelWho]

Minor mistake, at 8:23 instead of comparing the indexes of the left and right children, you should be comparing their values to get the smaller child!

[@adityamedhe]

that's not minor

[@acymiranda]

I don't know, but is getParent(index) correct? If I get the final heap of the example, like: [10, 15, 20, 17, 25] and I add an element in the end, for example, 32 and it would be below 17, so 17 is 32 parent. 32 index is 5. 17 index is 3. If we use getParent(5), I would have: (5 - 1) / 2 => 4 / 2 = 2 But index 2 is not 17, but 20... Am I missing something here?

[@dmitrybahtiarov3555]

There is error at 2:56, parent is (index - 1 ) / 2 and not (index - 2) / 2

[@CamdenBloke]

Pro tip: if your array is indexed at 1 (like with Fortran) the pointers are parent: (index-1)/2, left child:2*index, right child:2*index +1

[@xMercuryx56]

that's not pro, that's just math ... lmfao

[@octamodius]

Clean implementation. Clean explanation. Wonderful video! Thank you very much for taking the time to make this. I really needed it!

[@WorklLife]

This is one of Gayle Laakmann's best videos. She walks us through the code, array, and tree versions while speaking calmly in a pleasant voice. Thank you!

[@michaeldang8189]

hasParent method should be simplified to: hasParent(int index) {return index > 0}

[@randomrandom316]

Its quite clever

[@brianspinos]

Nice!!!

[@AyanSengupta17]

It should be (index-1)/2 for the parent and not (index-2)/2. Please correct it.

[@jkulkarn100]

At 10:31 , Parent should be Parent=(index-1) / 2

[@anwarshaikh6023]

Very nice explanation. Though including big O complexity of the operations would be great.

[@BryanTidwell2]

Anwar Shaikh insertion and removal should be logarithmic. of course poll is constant and search is linear but you wouldn't want to use the structure for search

[@damnstupidoldidiot8776]

It should be O(nlog(n)).

[@uusoft]

time complexity O(nlog(n)) space complexity O(1)

[@drophy]

Insertion and removal have a time complexity of O(log(n)), where 'n' is the number of nodes in the heap. This is because for example, during insertion, in the worst case scenario, you'll need to move the inserted element from the bottom all the way up. Therefore, the max number of swaps is the height of the tree, which is log2(n) approximately (note that this is just true if the tree is balanced, but they always are for heaps). For example, her tree had 10 nodes at some point, a height of 3 (or 4, depending on how you define 'height') and log2(10) = 3.32. The max number of swaps you might need when inserting is 3. The same idea applies for removal, since the element you place at the top might need to go all the way down. The space complexity of the structure is O(n), of course, since you need an array of size 'n'. The space complexity of the 2 operations, however, is indeed O(1), since they don't need additional space.

[@mvcds92]

The video feels incomplete because it never explains what a heap is used for, though the data structure very well.

[@jimmithfarrel8986]

A heap is used as a queue where the min (or max if max heap) is always accessed in O(1) time. If the min (which is always at index 0 is popped off, then the next smallest takes its place. Remember its stored linearly yet indexed logarithmically. Therefore its a "priority" queue.

[@mvcds92]

Yeah, I've googled it afterward, though it's kind of you helping me here, thanks!

[@musicprod8366]

Thank you : )

[@xNajda]

What's the difference then between a heap data set and just a normal ordered data set using a binary search for the placing of each new element?

[@sumitbhattacharya1720]

go read a book then.

[@Amolang991]

After you poll(), shouldn't you remove the element at `items[size - 1]`?

[@karanbopche]

has parent method is wrong. it will return true for index 0 which is the first node and does not have a parent. it should be like private boolean hasParent(int index) {return index > 0; }

[@jonasgrnbek7113]

That butterfly keyboard sound 😕

[@jimgetsjob9551]

so simple to impliment, just paste these blocks of code and *BAM* it probably does stuff but good luck explaining it on an interview

[@RagazzoKZ]

Very helpful video, but I don't like her voice

[@dmitrybahtiarov3555]

There is error at 2:56, parent is (index - 1 ) / 2 Or otherwise for left node we get (parent -1) instead of parent. Everything else is just brilliant, thank you for the Great Explanation! 💓

[@Itskfx]

Noticed that too, but I think they corrected that in their code implementation. lmk if I'm wrong cause I'm here for exam revision and I'm kinda weak at heaps.

[@maheshnampoothirikv5080]

We need to have one more line in the "poll()" method, correct me if I am wrong. I used the starting example (after inserting 3 as new value to heap) to test the same. int item = items[0]; items[0] = items[size - 1]; items[size - 1] = 0;//We need to make the last element zero explicitly as the last element will stay otherwise. size--;

[@dishantsheth9592]

The "size" variable maintains the boundary of the heap in the array and so there isn't a necessity to take care of elements with index >= size in the array. Also, "items[size-1] = 0" doesn't achieve the same result as assigning a dynamic node in a tree to null. Here, it simply gets assigned a value of 0. To help with understanding, consider the pop operation in the static implementation of a stack. The popped values remain in memory after pop but not in the stack because of the "TOP" pointer there. Similarly here, size keeps track of the boundary of the heap to help with add and poll operations.

[@AlexandruRepede]

this is useful for priority queues

[@tamoorhamid519]

That's one application.

[@jadanabil8044]

@@tamoorhamid519 what are the other applications?

[@tamoorhamid519]

@@jadanabil8044 They can be used to efficiently find the largest or smallest value in an array.

[@mgeasley]

that's a lot of "head starts". smh

[@tanveeralitapya7258]

Yeah!

[@kappamomondo1038]

To be fair, watching her type out all of those methods would be incredibly mundane

[@anirudhreddybasani3555]

In getParentIndex() function if childIndex=0 then (childIndex-1)/2 would evaluate to 0 as integer division will be performed... so hasParent(int index) function would return true even if index=0 as getParentIndex() function would return 0...So I think hasParent() function is not working when index=0....Is it correct????

[@imochurad]

It seems that your assumption is correct, however, you'll still gonna break from the while loop in heapifyUp() method b/c root is not bigger than itself. As a proper fix, one should check if the index passed into hasParent() method is 0. If it is -- return false.

[@anujpancholi8293]

hasParent or getParentIndex will not, and should not, work for index=0, since the root element itself is positioned at index 0, and by definition, it has no parent. You can include a check for this, but for the sake of simplicity, that check was not included here.

[@aswinbeats]

int getParentIndex(int index) { return (index - 1)/2; } The above code will return 0 when index is 0. Thus, hasParent function for index 0 will return true. I think that getParentIndex should be as follows int getParentIndex(int index) { return (int) Math.ceil( ((double)index - 2)/2 ); } Thus, hasParent(0) will be false because -1 is less than 0. Did I miss anything?

[@craigburkhart1616]

I was thinking the same thing. Even at value of 1 you get 0/2 which is dividing by zero. I have been doing this on paper so i havent run your code but it does seem to address the issue i was puzzled by. Thanks for posting

[@quantic7244]

Noticed it too, if I'm not mistaken this issue is easily solved by changing the condition in the method hasParent: getParentIndex(index) > 0, instead of >=

[@off_pudding443]

0/2 is not dividing by 0

[@WASDsweden]

hasParent can simply be index != 0

[@petersinsky9123]

then it would return FALSE even in the case when the parent has index 0

[@SuprStevn]

the function hasParent will return true for input index 0 (which has no parent). Unless I am missing something this is a bug. My code uses private boolean hasParent(int index) {return index >0;}

[@shawnjackson7568]

Space ooo yes this is a bug. Actually there are several bugs in here. Definitely the worst hacker rank video I’ve seen.

[@dvvdsvsd]

I have final exam tomorrow, after this explanation, if this will be my pick, I know I'm safe. Amazing videos!

[@utkarsh_108]

Best of luck

[@ShermanSitter]

I don't have an exam, but i found it useful as well! I don't know why, but heaps were so confusing...until now! :)

[@ShermanSitter]

@Chris Chu Learns ah shucks. thank you!

[@kenansari]

how it was?

[@Itskfx]

Same, I'm terrible at heaps. These vids help a lot!

[@alicianieto2822]

The video is awesome and what I needed to know. Thank you!

[@KelleyNielsenSalticidSoft]

At 9:03, she moves the updating of index to outside the else block. I'm thinking you could move both lines outside it and get rid of the else branch. Anyone disagree?

[@RathodDharmin]

It's more readable and less code, but "else" gives you an idea of flow.

[@mogomotsiseiphemo1681]

You would have to re-write the code as follows : if( items[index] >= items[smallerChildIndex] ) //Notice the change in the binary operator from < to >= { swap(index, smallerChildIndex); } index = smallerChildIndex;

[@ophir1982]

Possible error at 1:54 the algorithm is said to be swapping with the smallest of the 2 child elements (when bubbling down) So 20 is swapped with 4, then the pointer is swapped with 9 (left child) while the right child is 7 - smaller.

[@nopenope8409]

1 year later but that is not correct because what you see there is already swapped so there was 4 before the swap and 20 took the place of 4 and then took the place of 9. there isn't an error

[@LeaFox]

I read about heaps online and first implemented it using a right and left Node. I felt array, though - spidey senses. I wish I would have seen it on my own. But, this video was a close second. Thank you so much for a clear description.

[@comosehaceyt]

Thank you :D

[@kiaksarshirvanimoghaddam4354]

I always had problems understanding heaps, but you made it so clear. Thank you so much ...

[@AlexXPandian]

Best video explanation with code walkthrough showing how to answer the ubiquitous lazy interviewer question "Implement a Heap data structure".

[@minjipack9727]

1:54 said swap the value with the smaller child value, but the layer3 smaller value is 7 (7 < 9). Even in implementation part, it follows the same logic, so diagrams needs to be corrected as 20 should be under 9 I guess?

[@stackingflow]

elements are inserted in left to right order and yeah the 20 is below 9 only

[@ontimegrad7069]

Thanks for the video! But I am a bit confused about the smallerChirld at around 10 min. Should the left child always be the smaller one?

[@YuelengWang]

I think the hasParent implementation is tedious, and btw, the index = 0 also return true for has parent. ( (0 -1) / 2 = 0, btw, so tennically the tutor's implementation is wrong, but since we use it this way: hasParent(index) && parent(index) > items[index] which does not make much hurt because parent(0) == items[0] and it will always return False here), you can just tell if index has parent by just index > 0, because every node has parent except the root which has index 0.

[@DarthTofu2]

Maybe I'm getting too far into the weeds, but how is this heap arranged? What are the times for number placement? Oh I were searching for the value 9, what rules would I use to run from the origin to that value?

[@darwinnacionales13]

I think the use of heap is to easily access either the minimum or the maximum value whenever you call the poll() method. If you need to be able to quickly search by value, you should be using BST.

[@AMANDHOL-f7v]

Quick question: Why do we have to compare with both 2:03 , left node and right node, because aren't left nodes intuitively supposed to be smaller than the right node?

[@xXmayank.kumarXx]

Heaps can be thought of as a binary tree. Peek takes O(1) while other operations take O(log n). For min heap: 1) Insert new node at last, then heapify (ie swap with parent until parent > child) 2) Delete the root node, replace last element at root then heapify (ie swap down)

[@mattak77]

private int getParentIndex(int childIndex) { return (childIndex - 1) / 2; } private bool hasParent(int index) { return getParentIndex(index) >= 0; } looking at these method, if I pass 0 to hasParent, it returns 0 because getParentIndex ==> (0-1)/2 returns 0 hasparent 0 >= 0 return true. But if an element is at index 0, it is root and no parent so the method should return false. What am I missing here?

[@ijustsawthat]

Do you know why Interviewers love questions about Trees, Graphs and Heaps ? Because to answer those, you have to write so much boilerplate, your interview time is gone. See how much she had to past in this just to start writing the actual solution.

[@SicariusVIII]

Really have to appreciate the readability of the code a variables.

[@trampflips101]

surely it's (index-1)/2, not (index-2)/2. The former would give 2 as parent for 5 and 6, whilst the latter would incorrectly give 1 as parent of 5.

[@SpiritVector]

My brain just short circuited, I think I might be stupid. So if to get to a child of an element you have to scale the index by two and then add one of two choices {1, 2}... but to do the reverse you have to (index -2) then divide that by 2? But what if the index is 1?

[@felixg4785]

I wish I had friends like her.

[@arianazin5419]

No please.

[@AbdulmajedGamer]

I wish I had friends.

[@ibrahimkassem7505]

There is an error in the code of getting the parent index, it must be "childIndex / 2 - 1" other wise it will be stuck in the root. or we can put in the while loop (hasParent(index) && index != 0

[@listigertrex8923]

Can someone explain me please why 50% of the internet says parent is (n-1) / 2 and the other 50% says its (n-2) / 2 in a Min Heap? For me when I do the calc (n-1) / 2 should be right.

[@RobsRobotChannel]

At 6:45 it seems like she swaps the value of the parent with the index itself, rather than the value of the array at that index location. Should it not be: swap(getParentIndex(index), items[index]) on Line #56??????

[@zaretix]

No. the swap method swaps the items at the two indices provided as arguments in swap().

[@David_389k]

Emotionally, we are thrilled to announce that the confirmation of your Sales Incentive payment has been processed.

[@technowey]

Thanks you for this excellent video. It''s the best, most concise and straightforward, explanation of a heap that I've seen.

[@BobBigWheels]

Simple explanation. I still don't understand why anyone would use a heap over a BST. Is it because it is much simpler to add and get the extreme value that is built for? What contexts is that useful?

[@Allyourneedsmet]

If i said I understand anything, I would be lying... Shi

[@H3000-v7i]

"Now what about removing the minimum element" Maybe rather start with removing an element in general. What are the rules here? These videos got potential and start out well, until you fuck them up! Either with mistakes in code, misuse of terminology, or forget to mention essential things. What a shame!

[@aanchalsharma8362]

Why do I relate to that node that is all empty inside! xD

[@cicakmuhamed]

At 1:38, did you mean "... we swap that value at the root with the last leaf node"? Because that is certainly not the same as swapping with the last element added. I find that confusing phrasing. Other than that, it's a great tutorial, thank you.

[@Itskfx]

Can anyone how'd it have to change in order for it to function as max heap? Currently I assume I'd have to use heapifyDown in the add function instead of heapifyUp?

[@exoexobtsbts2569]

Worst thing you can do when teaching programming / computer science is continuously copying and pasting code out of nowhere. Terrible tutorial.

[@whiskas-1]

There is an animation on 1:49 which not reflects the actual logic of the heap sink down ( accordingnaly to the idea of swaping with smallest child node when bubbling down )

[@programmercouple]

This is the best explanation of Heap. Thanks 🙌🏻

[@lolipopscandy62]

How does this not have more views?? What a simple, and amazing explanation. Subscribed!!!

[@palanisamymadakkannu4350]

only entertainment videos ll get more views.. useful videos wont get..😊

[@intrepidsouls]

Agree with you. I watched quite a lot of her videos and it seems like she doesn't quite understand what she is talking about either.

[@blasttrash]

@@intrepidsouls I agree too. Her book is good though.

[@thatoneuser8600]

Because it doesn't answer why heaps are used or when one should use them. It doesn't give a perfect concrete use-case where a heap would always be beneficial if used.

[@devinebug]

What is the time and space complexity ? And what are the effective use cases for min/max heap?

[@akhiljain1695]

I was searching for something just like this. Awesome explanation of implementation of heap.

[@Daniel___9v2]

By the unfortunate turn of events, a system error led to the transaction being sent to an invalid email address.

[@lifewear.reseller]

Can someone explain me why don’t we just add 25 at the end of heap instead of deleting 10, because we just wanted to add new one not deleting the min one...?

[@Bennettone]

Please fix error in parent index formula

[@manuelhe46]

The link to the coding interview tutorial is broken. It gets me a 404. I'm looking for the source code

[@motivationalcomred]

3:28 getleftchildindex

[@itsmewaqar]

Am I the only one who watch all of her videos on 0.75x speed ? Man, this girl speaks fast.

[@IMC5776]

Whats the intuition behind choosing the right child over the left child? Why do we choose the right child if it is less than the left child? A heap is not necessarily a binary search tree is it?

[@carlosl5820]

I wish my data structures class at uni was as well explained as this video, my professor sucks

[@pranavnyavanandi9710]

Why do the heapify() methods need to be public ? Any reason a user might want to access them?

[@abdulrashid1060]

Secret of Learning Heaps; 1) Heaps = Arrays. (Start with Index 1) 2) Parent of Current Index = Current Index / 2 Where to Put new data? 3) Current Index Left Node = Current Index * 2 4) Current Index Right Node = (Current Index * 2) + 1

[@devin6977]

Why would you want the heapify methods to be public? Is there a need for them to be accessible outside of the class?

[@this-is-bioman]

Why didn't you use Kotlin for the implementation? Java is so verbose :|

[@GloriaPartin-o3e]

Hall Elizabeth Thomas Karen Robinson Jose

[@skylinefx049]

3:05 flashbang warning

[@BeginningProgrammer]

This is a really nice explanation of min heaps.... Very nice, very clear, very simple , concise and short enough to pick up in a jiffy. Thanks Gayle.

[@beingyourself9824]

Today I actually understand how coders actually codes and how to actually maintain the semantics of variables name fabulous explanation I sub you within 1 minutes of this video

[@ShermanSitter]

at 1:50 if it gets swapped with the 'smaller' of the two, why does the 20 get swapped with the 9 instead of the 7?

[@stackingflow]

+1

[@gutenmorgan2063]

i know you wrote this comment a while ago but I believe we do it that way because we have to go to the next available spot. If you write horizontally on all of the nodes starting from the root at 0, where she does in the video would be the next place you would go as if you wanted to add nodes, that would be the first place in terms of heaps. I hope this is helpful and that your figured it out!

[@felica4474]

the index of node parent is (index-1)//2, not (index-2)/2.

[@NickeManarin]

At @2:49, isn't parent = (index - 1) / 2 like the code uses ?

[@DominicVictoria]

What overhead will you get from an array of class?

[@nobodycaresmeself]

her voice tone is like someone who is used to be reject candidates due to they cannot reverse a binary tree

[@cameronswartzell1362]

Not a Java programmer so I don’t understand how “swap” works in the heapify up section at 6:45. She has swap(index1, index2)… but it’s not a method of any particular list or be or object, nor is she passing it a list. How would it know to swap the values of a particular list? Looking up docs it seems like this is just an error; the swap function is swap(list, index1, index2)

[@gbh1998]

Hi Cameron, the answer is actually Object Oriented Programming. If you see the beginning of code, she has declared the array as what we call in Java an instance variable, which belongs to a class (also called members of a class), a blueprint/template of an object. So, with this, we can use our instance variable array in one of methods which are also members of a class. So, if the array is a member of a class, there is no point in sending it to one of the parameters in methods (in functional languages like C, they are functions) The swap method you have mentioned does not belong to a class, so we need to pass the array data structure to the method parameter. Hope I able to clear some doubts Check out Object Oriented Programming, more doubts will be cleared.

[@gutenmorgan2063]

5:24 - why are we removing the 10? what is the point of removing the minimum element?

A nice explanation, but I find the vocal fry unbearable to listen to.

[@gauravmaithani4707]

best vid ever... thanks McDowell.

[@Amolang991]

what is the reason you set the helper methods as private and others for public?

[@roman-berezkin]

The best explanation of heaps ever, it got me first try.

[@sandole97]

Can I be wild and ask why Gayle has a Korean coding book in the intro?