How To Reverse A Singly Linked List | The Ultimate Explanation (Iteratively & Recursively)

[@BackToBackSWE]

Table of Contents: (This was one of the first videos I made. Audio sucks. Lighting sucks. It is pretty bad. I'm sorry.) Intro & Video Setup 0:00 - 0:33 Problem Introduction 0:33 - 2:00 Iterative 2:00 - 6:08 Recursive 6:08 - 9:42 Recursive (Detailed Walkthrough) 9:42 - 13:26 Conclusion 13:26 - 13:49 For the recursive way forgot to stress the line saying "head.next = null", this ensures that you point the last node in the reversed portion (built slowly as we go upwards in the recursion) to null. This is important.

[@mrknight411]

As for the recursive solution, I believe people are confused about head.next.next=head and head.next=null operations because they're not seeing that both rlh and head are referencing the same mutable node objects in heap. In stack where head.data is 3: rlh is 4 head is 3->4 head.next.next=head is 3->4->3->4... head.next=null is 4->3, since it removes all "next" obj ref in 3 to 4, leaving intact 4->3 Since 4 now points to 3 in memory, rlh becomes 4->3 ** debug the recursive solution in your IDE and place a breakpoint on line head.next=null. You will see that rlh.next is now node 3. In stack where head.data is 2: rlh is 4->3 head is 2->3 head.next.next=head, which is 2->3->2->3... head.next=null is 3->2 2 Now rlh is 4->3->2 2 And now the main stack: rlh is 4->3->2 head is 1->2 head.next.next=head is 1->2->1->2... head.next=null is 2->1 , since the obj ref from 1 to 2 is removed Now rlh is 4->3->2->1 , since 2 now points to 1. Recursion is not easy to wrap your head around, especially when dealing with references (or pointers). You did a good job explaining the stack, but you missed the heap :)

[@sarfarazalam6077]

Yeah, I was also wondering why use "head.next = null". Thank you, for clarifying this! Thanks for the video!

[@BackToBackSWE]

yo

[@irfaanjamarussadiq5500]

Honestly I didn't really mind audio/lighting quality. But I think you could speak a little slower, because it kind of stresses me out when you speak so fast and it's also harder to understand that way. Beyond that, this was a really nice explanation!

[@BackToBackSWE]

@@irfaanjamarussadiq5500 old video

[@5astelija75]

I never imagined learning code would be this intense

[@WildThoughtsAI]

lmao

[@joshuamarcano350]

🤣

[@Working800]

I like your comment, it is a bitter true a few people discuss lol

[@anitasingh-jf1ql]

This is literally the best explanation of reversing a linked list on youtube!!!

[@BackToBackSWE]

thx

[@VickyMei]

lol I love how you are so enthusiastic about this :D

[@BackToBackSWE]

ya

[@stinkykyle96]

You seem so excited to explain the problems, it makes your videos engaging 😁 But honestly this is some of the best interview prep that I've gotten, especially since I'm a visual learner.

[@BackToBackSWE]

Great! :) -Ben

[@didi_10242]

I have been preparing interviews for data scientist positions, and the coding part of the job is not too easy for a person without a computer science background. I was struggling in understanding the recursion function for a really long time, and this is the first time I finally understand it intuitively after watching your video. Thank you so much! Your call stack illustration helps a ton!

[@BackToBackSWE]

Thank you, glad you liked it 😀 Do check out backtobackswe.com/platform/content and please recommend us to your family and friends 😀

[@fardin_shaik]

No one can explain recursive thinking better than you, man. Respect

[@BackToBackSWE]

hahaha thanks

[@jiadongyu131]

I found myself smiling as I watched this video because your passion and enthusiasm really shone through. Great explanation and good work!

[@BackToBackSWE]

Great to hear!

[@sky76570]

12:58 Hahaha, explanation with synchronized movements is hilarious bro. Respect.

[@BackToBackSWE]

ye

[@Mines2013]

Never seen anyone this excited about reversing a linked list, haha

[@BackToBackSWE]

I am a lively guy

[@taesookim531]

Thank you,, NEXT

[@BackToBackSWE]

hahahahahahahahahaha. Best comment on this whole channel. I love you.

[@ApexPlayground_divine]

Bro thanks alot man I final understand this stuff, thanks so much😭

[@JakealiciousAndBros]

I’ve been struggling with DSA, but the way you taught this really opened my eyes. I’ll admit, I watched a few parts several times, but I love your energy and will definitely be returning for future problems!

[@cf921]

This guy explains reversing a linked list like playing basketball with all the shoes' sound lol, love your enthusiasm ;)))

[@BackToBackSWE]

hey

[@jinhuang7258]

Explains recursion so well. Thank you.

[@AndrewWheelerJr]

Kind of makes me miss college. I would also get really excited when I finally understood something and was explaining it to someone. Thank you for helping me understand it.

[@BackToBackSWE]

Sure. great.

[@Sheldor.The.Conqueror]

You explain recursion like no one in their wildest dreams can even think of !!!!

[@eg816]

you're so good at explaining!. thanks !!

[@BackToBackSWE]

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[@nicolerussack9800]

Idk how you can be so excited talking about recursion but it's making me excited!!!

[@yuselina8769]

Really love your illustration! Thank you!!

[@rikki146]

bruh this is a treasure on youtube. thank you!

[@pranavbhat92]

The intention & intensity with which you explain is commendable. You deserve more subscribers. I subscribed! 😊

[@roliverma5851]

Best video on recursion I have ever watched

[@uditgaba1450]

saw this video for reversing a linked list recursively . Helped me a lot . Thanks man !

[@BackToBackSWE]

great - sure

[@robinr2015]

Does anyone know why next can't be initialized to curr.next? When I did it that way I got a null pointer exception but I can't figure out why that is the case

[@robinr2015]

nevermind, I realized it is because I am changing next after making curr point to prev, so next was just getting assigned to prev

[@BackToBackSWE]

@@robinr2015 great to hear you got it!

[@Polarwhite43]

Best explanation out there Thanks man

[@BackToBackSWE]

thanks

[@zahranajib5528]

Ahhh please make more videos!!! You are such a lifesaver I swear. Watched your video like 5 times to really understand the concept. Maybe on recursion and more data structures. Also, a suggestion, try to talker slower :)

[@BackToBackSWE]

Thanks and we will

[@fantasy9960]

this is the best explanation for listNode reverse! Really appreciate it !!

[@kunalkeshavsinghsahni6861]

Thanks for the beautiful explanation

[@BackToBackSWE]

sure

[@serrasilva7316]

I love your enthusiasm. It makes it so much more fun to learn.

[@prathyyyyy]

THE BEST VIDEO ON KZbin !!!!!!!! IM IN LOVE :)

[@bn2005]

thank you for framing this as pointer manipulation instead of "moving nodes" like so many tutorials do

[@BackToBackSWE]

sure

[@myeoIgong]

Thanks buddy, I like your explanation better than other's.

[@BackToBackSWE]

nice

[@ashwinvarma9349]

I came here for the recursive method and the explanation was great! keep it up!

[@BackToBackSWE]

sure

[@maricampbell8240]

Thank you! The iterative explanation and diagram was so helpful for me. I was able to implement the function without looking at a code example, but from imagining the diagram you created.

[@BackToBackSWE]

nice

[@anthonysoffice9367]

you explanation on recursive method was pretty clear

[@BackToBackSWE]

great glad it helped

[@soujanyayprabhakar]

I watched so many explanations regarding this i could not understand but i got an idea about this by watching your video . Kudos !!!

[@BackToBackSWE]

thanks

[@deepeshrathod2044]

Damn! this is the best video I found. It just cleared all my doubts for reversing a linked list recursively. Thanks for the video.

[@anmolsharma9539]

The time i see that you had made a video on this i know that my doubt is going to get resloved before the video gets over and it happened :)

[@joshj8459]

Yea I subscribed. You had me on the edge of my seat the entire video like I'm watching a suspenseful movie.

[@BackToBackSWE]

haha thanks for the sub

[@showmerockk]

this is the best explanation of this concept

[@BackToBackSWE]

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[@ExplorerSpace]

omg you are a life savior

[@LesAxolotl]

Hey man, thanks for this video. It's such a simple an elegant solution. I spent the last 12 hours trying to solve it myself (not too smart really) and finally gave up and came here. Your explanation is so clear. Cheers!

[@aniarablechannel4668]

Just because you couldn’t solve it doesn’t mean you are “not too smart”. It could just be a difference in coding experience. I wouldn’t get down on yourself. You’re already doing great and setting yourself up for success by working hard to get better and watching videos like this!

[@alhaithamaljabri6126]

OMG... Thank you so much for your amazing teaching skills! I am happily crying since this finally makes sense to me after all that struggle...

[@BackToBackSWE]

thanks and nice haha

[@himanshusarad6491]

tysm man ...you literally nailed it with this explanation .....finally i understood how to do this now.

[@teensymarie]

I can only hope that one day I can explain this with as much excitement and clarity as you do.

[@jbphoto7]

Finally! A perfect explanation.

[@Ryan-xb1ry]

I finally understand this. Thank you.

[@BackToBackSWE]

Thank you, glad you liked it 😀 Do check out backtobackswe.com/platform/content and please recommend us to your family and friends 😀

[@labgeek1559]

love your videos, keep up the great work.

[@BackToBackSWE]

thanks

[@anupamdungdung9417]

Sir the way you give so comprehensive explanation... I love it!

[@supamdeepbains5172]

my homie always there to help me with data structures

[@niharikayerramsetty8582]

i have seen many videos but urs is awesome and the best

[@BackToBackSWE]

thanks.

[@kanishkarao979]

Finally a simple explanation. Thanking God for creating you 😁 😁 😁 😁 😁

[@chamnil8666]

Thanks Handy Dandy,now i can reverse a linked list,have a blessed day.

[@BackToBackSWE]

great

[@mamoodshaban3119]

That was really helpful man. I'm grateful! Keep up with the good work!

[@BackToBackSWE]

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[@TadiwanasheNyamapfene]

Absolutely the best video to explain this. Just took away all my frustration lol!!

[@jake18111996]

every one is asking how he is so enthusiastic but my mannnn took a preworkout and he is getting the pre workout kick!!!!

[@helenashatalova7828]

Finally, I got it! Thank you so much for your video! Your channel is a hidden jam!

[@BackToBackSWE]

thanks!

[@Roover_sa]

Brother no words for you . i am stuck at temp->next->next but you nailed it with the example "Let me Add myself" Lot's of love from india

[@BackToBackSWE]

hey

[@tienduong7729]

My man big on energy. Love it!

[@iraisvalenzuela6147]

Thanks for the video! You simplified this so much for me!

[@rohitmehta5258]

So once the base case is hit (head becomes the 4th node, reverse happens and the list is now 5 -> 4), the return head is applied to every instance of the function in each stack right? So first the return head is applied to the instance in the stack where the reverse list head is 4 (producing 5 -> 4), then to the stack where reverse list head is 3 (producing 4 -> 3), so on and so on. My question is, how does the program know to apply the return head to the instances in reverse order?

[@BackToBackSWE]

What do you mean? Sorry, I'm not understanding - all the data is saved in the stack & each stack from has memory of the head that was passed into its call

[@jacktrainer4387]

These videos are amazing. You're a phenomenal instructor.

[@BackToBackSWE]

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[@ArjunKalidas]

Wow! Leetcode had only 250 problems when this video was recorded. Now it has crossed the event horizon lol!

[@BackToBackSWE]

No it had 780, I just aimed to do 250 problems that hit core concepts since most of their problems just repeat core ideas.

[@romanazeem3098]

@@BackToBackSWE Hey man. it was the best explanation for the reversal link list. it doesn't take a second try.

[@ArjunKalidas]

@@BackToBackSWE True, that makes a whole lot of sense.

[@tarunkolla]

I got the logic by 3:20 you are really dope at this.

[@BackToBackSWE]

haha thx

[@RizaHariati]

WAAAH I FINALLY UNDERSTAND IT BROO!!!

[@BackToBackSWE]

great

[@gazalaparveen1254]

This is literally the best explanation I have come across🙌

[@BackToBackSWE]

Thank you, glad you liked it 😀 Do check out backtobackswe.com/platform/content and please recommend us to your family and friends 😀

[@ri7_cars]

Thanks, finally understand the recursive solution. The explanation of call stack really helps.

[@BackToBackSWE]

thanks

[@annawilson3824]

10:40 so we have head that points to 3 and new_head(reversedHead) that points to 4. Overall, since each time we have a function call with a new/subsequent head, once we start returning, the head is going to be updated for each call in the reversed order once we propagate back/up the stack, but new_head/reversed will stay the same (we set it value once when we reach the depth of the calls) and always points to 4, so we return it at the end of the whole function.

[@TCErnesto]

I really liked your explanation on the recursive method bro thanks

[@lukehatcher98]

incredible explanation. THANK YOU

[@normantas3940]

Not ganna lie, this video was very clear!

[@BackToBackSWE]

nice

[@soradev5386]

It took a while to sink in, but thanks for this detailed explanation to the problem. You killed it

[@BackToBackSWE]

thx

[@txaggie09]

Very good explanation. I've watched a few of your videos and all of them so far have been easy to follow and does a good job of explaining the approach.

[@BackToBackSWE]

thanks, welcome to the tribe 🍖

[@1cannon3]

So is ReversedListHead returning: x

[@BackToBackSWE]

'reversedListHead' just preserves the head of the reversed linked list as we return back upwards and pop off stack frames. The actual reversing is done on the 'head' element where we point the 'next' field of the node after 'head' to 'head' itself (a fundamental reverse operation). We do this repeatedly and the very nature of popping off stack frames makes us do this in a manner traversing the list backwards...leading to a full list reversal after doing that subroutine for each link. "My understanding was that the recursive call keeps getting called before the things underneath it ever get called until you hit the base case." Yep. We keep calling reverseList(head.next); until we hit the base case...then we have all nodes on the stack with recursive calls. "But then what?" The 2nd bottom call now gets back the head of the reversed list in "reversedListHead". Then the node we hold called 'head' (in the 2nd to bottom call) points it's next node (the last node in the list) to itself. This continues upward. Every call hands off "reversedListHead" upwards to the next guy until the top level call hands off that reference to the original caller (and in the meanwhile the whole list got reversed by the work in the middle). Is this clear? Continue asking questions.

[@1cannon3]

"'reversedListHead' just preserves the head of the reversed linked list as we return back upwards and pop off stack frames." "The actual reversing is done on the 'head' element where we point the 'next' field of the node after 'head' to 'head' itself (a fundamental reverse operation)." This was what I needed, so clear! My mistake was forgetting that all the heads were already attached to our now reversed linked list as we go back up. Thanks for the quick reply, you're incredible, wish I had your work ethic!!

[@BackToBackSWE]

@@1cannon3 Excellent. I have "ok" work ethic, trust me. It is just that this stuff pisses me off...that's why this channel exists. To be a little help.

[@sanghunlee6825]

Life saver bro Thanks for your kind explanation

[@alpavaidya412]

If I had a teacher as enthu as u bout teaching I would have been better programmer today...appreciate your efforts

[@BackToBackSWE]

thanks

[@spiritualgyanhindi2681]

Thnx! Finally, I got this logic of returning back after top to bottom😄

[@andywu9774]

This guy is probably the most passionate CS guy in the entire universe!

[@mingyuetang7041]

Great explanation!!! gonna use your videos for interview prep. Many thanks 🙇

[@BackToBackSWE]

great!

[@wilsonxie]

THIS IS WAY TOO GOOD! HOLY COW

[@BackToBackSWE]

thx

[@kevintravelsgram]

I thought the recursive way was really confusing so I had to look it up on KZbin. Really glad I found this video. Amazing explanations!

[@BackToBackSWE]

haha nice, wassup

[@mahfxd]

Bro the energy, i love it

[@diegoi1313]

Thank you for this well informed video!

[@phantom4dji466]

this was awesome and helped me out a lot. Thank you!

[@darod6098]

Subscribed. Keep uploading videos like this everyday and your channel will grow a lot! You rock

[@BackToBackSWE]

Workin' on it. I'm tiring haha

[@darod6098]

@@BackToBackSWE Haha you can do it :) Thanks for your efforts!

[@jacobl7451]

this problem is basically a meme “after years of tech experience. I got rejected by Google....couldn’t reverse a linked list”

[@jaiprakashshrivastav2378]

Very good diagrams and explanation. Keep it up buddy !

[@BackToBackSWE]

thanks

[@raz478]

You explained it very well

[@parnitasharma8898]

thanks man!...U saved my time and energy

[@nabilkhalil6935]

So helpful thank you so much.

[@BackToBackSWE]

sure

[@tony78545]

you're amazing dude! Thanks a lot!

[@stumbino8361]

Keep up the good work! Doing a great job on the content.

[@harshalpatil5523]

you explained amazing brother

[@BackToBackSWE]

thx

[@samuel2318]

really love his way to explain this question🤣 he is so enthusiastic and energetic!

[@leetsai6220]

Watching your explanation and just the passion that you put into these videos really helped me remember the nuances of this problem.

[@BackToBackSWE]

great

[@ubohllc6587]

Great Job and great explanation. You take so much effort to explain. Thanks so much!!

[@BackToBackSWE]

sure

[@congdatt]

Amazing Teacher 🥳

[@BackToBackSWE]

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[@ankurjain8339]

Such a thorough and simple explanation. Thank you!

[@BackToBackSWE]

sure

[@fabregasonline]

what happens if you do not make current.next to prev, won't it still be the same

[@BackToBackSWE]

I dont remember the code much tbh - I'd contribute critical thinking if I did