Binary Trees in Python: Introduction and Traversal Algorithms
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- Опубликовано: 11 мар 2018
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In this video, we will go over the binary tree data structure. We then go over how to implement this data structure in Python. We then cover the three recursive depth-first search traversal algorithms (preorder, inorder, and postorder) and implement those recursively in Python.
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Note that @26:30 I call the "inorder" traversal function inside of the "postorder" traversal function and neglected to change it to postorder. This was a copy-paste error. You can fix it by changing the "inorder" call in postorder to the appropriate postorder call. Sorry for the mistake, and thanks for watching!
How can you go from child back to parent if you don't have a next and previous pointer?
By next and previous I should say if each node doesnt have a pointer pointing to the one above it as well as to the one below it?
@@Soccercrazyigboman he is using recursion
That changes the expected output from 4-2-5-6-3-7-1- to 4-5-2-6-7-3-1-
Hi, would be more efficient to add a function from within the class for adding values to the tree? does it help/ would it be more efficient especially during interviews
Great Video! Thank you for visualising and really emphasizing on the most basic aspects as this is something which I personally as a Beginner seem to struggle with!
Keep up the good work it is really appreciated!
Thanks, Chris! That feedback is really helpful for me and I will try to keep that in mind for future videos. Thanks again for watching and for the comment!
Hands down the best explanation, I've struggled for so long trying to get the intuition through my textbook. Thanks!
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@@LucidProgramming Sure, I've subscribed and signed up for the mailing list. You've made my interviewing process so much smoother.
@@safderaree Awesome, that's really great to hear! :)
Very informative and easy-to-understand tutorial. Thank you for this incredible contribution to the enthusiasts and learners!
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Большое спасибо за ваше видео)
Это потрясающе, я посмотрел ваш Гайд по связным спискам и теперь я тут. Лучшие видео по этой теме!)
I was stuck for hours trying to understand this, you made it so simple! Thank you so much
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@@LucidProgramming Done and done my good man!
@@turjo119 Cheers, thank you!
Your explanation to data structure concepts in python is THE best!
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i really love the way you are explaining all these concepts in very effective manner thanks for this :)
Hi Sarfaraz. Really appreciate that comment, that's very much appreciated! Thank you for watching, and I'm happy to hear this video was effective. Cheers!
Thank you for the great tutorial! I am a new developer still looking for a job after graduation from a boot camp. I never had a chance to learn the data-structure, dfs, bfs etc before. Now, it's getting clear to me step by step after watching the videos you posted.
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This is the most simplified video tutorial with good examples, the best I have seen so far. I got everything after watching this video only once. A lot of online tutorials are just too ambiguous and really confusing. Lucid you're the best for sure. Thanks a lot
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@@LucidProgramming Sure, I'm interested. But how do I subscribe to your mail list please?
@@muhammadsalisabdulsalam4016 Just follow the link here: bit.ly/lp_email
:)
God bless you Sir. I have been following your tutorials for a while and you are a LEGEND
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Too excited to sleep now! I finally understand it! Best video on Traversal Algorithms explanation! Consider watching all your python video!
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@@LucidProgramming Just a tiny recommendation, on 17:06, it would be better to use self.root instead of tree .root. Keep going! !!
@@enkaibi2756 Point well taken. Thanks for the comment! :)
Thanks for such a cool implementation explanation.
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Awesome tutorial! Thanks.
Thank you, Philip! I'm glad you found the tutorial useful! Cheers, and thanks again for watching.
Really good tutorial man! Thanks
Thank you, David, I'm very happy to hear that you enjoyed the tutorial! Cheers.
Best video I've seen on this topic. Thank you!
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Beautiful presentation.
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Amazing explanation, thanks!
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Thank you so much for the great tutorial!!
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Hi there!
Thank you so much for this! I just discovered your videos and they are so helpful! I was struggling a lot to understand how I could implement and traverse a binary tree. There are countless videos about this but yours really made me understand and gave me hope that with time and practice I could learn these concepts.
I saw you have videos on many data structures and I plan to go through all of them and try to understand. Could you please also cover graph implementation and traversal in Python?
Thank you again for your help!
Wish you the best!
You can feel free to make suggestions on my Patreon page. This would streamline what you want to see and also help to support my channel.
www.patreon.com/lucidprogramming
Best Explanation
recursion is bit confusing but following each step through example made it easy... Thanks
Awesome, that's great to hear. Thanks for watching!
Great explanations! Thank you very much!!
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Dude thank you for this!
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wow - it is awesome !!!
i can tell one analogy that helped me a lot : when do i collect the value :
• pre-order - when value collected ?: once I am hovering on a node I am collecting it's value.
• in-order - when value collected ?: if the node has no left node or the left node is already visited then add this value.
• post-order - when value collected ?: once the node has no left nor right or left and right has been already visited then I will collect the value.
HTH
Thanks, and yes that's a great way to remember them!
Great video. Thank you!
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Amazing explanation well worth the time to watch.
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I appreciate the good video and clear explanation. With a little more editing and attention to detail, it can become more polished and will be excellent.
Thank you! I hope that the videos on this channel keep getting progressively better. Thanks for watching!
Thank you! Explained it very well.
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Hi, thank you so much for the video. I am a little confused still about one thing. When in the preorder traversal, when it comes to a leaf that dosn't have a left or right node, in the code how does it go backwards to the previous node or back to the root?
I would encourage you to check out my playlist on recursion, as it seems you have a gap in understanding how recursive calls are treated.
Awesome! Just Awesome! This is where I could navigate to learn such an important thing earlier.
Thank you! :)
phenomenal video, thank you so much!!!
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Great work very easy to understand i really appreciate what you are doing to help us
Hey Huru. Thank you very much for that comment. It's really great to hear that these videos have been helpful to you. I really get a lot of enjoyment from doing so as well, so the benefit is mutual! Cheers and thanks again for watching.
Just 30 minutes i go from nothing to understand a lot more about tree could you please make a tutorial about graph and some algorithm about tree ?
Really happy to hear that. Yes, graphs are most certainly coming up soon! Thanks for the suggestion and for the comment!
Can't be explained better. Thank you!
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@@LucidProgramming Done! :)
Are you during Mock Interviews?
Or know someone who does?
@@ilanaizelman3993 Thank you, Ilan! Your support is very much appreciated! And yes, I do perform mock interviews, coaching, and consulting services for software-based job preparation. If you're interested in setting something up, you're free to reach out to me in the "About" section of my channel and we can coordinate from there if you like. Cheers, and best of luck in your preparation in any case!
Could you include the link for the presentation as well.It serves as a quick recap.Thanks for the great explanation.
At the moment, those slides are a bit all over the place. However, it would certainly not be an issue to clean them up and post a link in the description of this video. Might take me a while to get around to it, but I'll add it to the list. Thanks for the suggestion and thanks for watching! :)
great explanation!!
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Great Stuff! Made it very easy to understand! Thanks!
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@@LucidProgramming sure!
@@hussainaqeel7371 Cheers, thank you!
Thank god I looked at the comments regarding the issue with the post order function, I spent a good 10 mins looking to see if I had the wrong implementation
Glad you caught that, and sorry for the trouble!
Amazing video. Thank you very much. I just have one question. Is it possible to get a string of the tree using the inorder function but with only using one argument instead of two?
Sure, you should be able to do that.
Very valuable explanation! Thanks : )
Glad to hear it, and thanks for watching! :)
@@LucidProgramming The preorder_print function is the sort of thing I would have had a hard time coming up with myself. My naïve attempt would have been to have some list or dictionary of nodes whose left child you've already visited ... instead you simply wrote (amongst other things I'm skipping)
traversal = self.preorder_print(start.left, traversal)
traversal = self.preorder_print(start.right, traversal)
This is really interesting, it's orders of magnitude more elegant and succinct. I wouldn't have come up with it. I would have written some longwinded solution using an extra list or dictionary.
Do you know a book or online resource that contains Python implementations of many standard data structures ?
@@olivierbegassat851 Thanks for the comment. Honestly, you can probably just Google for Python data structures and find something reasonably good.
Thank you soo much for this!
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Dude you earned a subscriber and a fan
Thank you, man! Sincerely appreciate the support and love! Cheers!
Extremely thankful to you sir.
Thank you for the kind words, glad the videos are useful for you!
@@LucidProgramming A very welcome sir. Please keep providing. The growth doesn’t justify ur quality. Keep it up sir.
Legend perfect explanation thank´s I like it.
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Nice introduction! Thanks to support the TI community.
Cheers!
good clarification skills, gave it a like
Very much appreciated--thank you for the support!
All Superheroes don't Wear Capes. Some wear Suit too :)
Haha, well I hope the videos are helpful to you. Cheers, and thanks for watching!
can you explain @19:33 when you call return tranversal, why it returns you the previous node of the current node that has empty left and right childs?
Hi Hakan. Sure, the reason for this is because as we hit the base case of the function, we start working back through the stack frame from all the previous recursive calls to the function. I would recommend you print out the output as these calls are being made to see when and where they happen. I think seeing that can be helpful. Hope that makes sense, and thanks for watching!
Thank you for this video
Your videos deserve more views
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great video! i would suggest matching the colors of the slides to the colors of the editor, because the sudden switch from dark to light doesn't sit well while watching. thanks!
Thanks for the feedback, that's very helpful to know! Thanks again for watching as well!
no problem dude, i really appreciate your videos!
@@gargoth Cheers, thank you!
I'm only 3 min in but love the explanations. Thanks!
Great to hear, and thank you for the kind words!
loved it!
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Thanks! Helpful :D
Thank you for watching, and glad to hear that it was helpful!
Hi..Thank you so much for uploading video with nice explanation..
Cheers! If you found the video helpful, I would definitely appreciate any a like/subscribe/share! Thanks for watching!
very good video for those who want to pick the grasp of binary tree quickly
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This is fantastic, thank you
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@@LucidProgramming Already done!
Just a small question, is it necessary to make whole class BinaryTree just for the root node? or it has something to do with python versions?
Hi. what do you think about it? https: //www.youtube.com/watch? v = xdmijinrk-w
amazing video
Hi Vincent, excellent as usual. is it possible to traverse iteratively as well? you got some code for us to see?
Of course, anything you can do recursively you can generally do iteratively. I don't have code for it, but I encourage you to try.
These are awesome videos I have ever seen. I have a question, why don't you use self.root in print_tree() function, but use tree.root?
Thanks! And either convention is fine there. Using self.root would work just as well. Cheers!
@@LucidProgramming Thank you a lot, these videos would definitely help me in my future career interview! I will recommend it to other friends.
@@jingfenghong2312 Thank you, I sincerely appreciate that!
wish i found your channel earlier. very good
Thanks, TJ. I'm happy you stumbled on it now, and I hope the videos serve you well! Thanks for the comment, and happy coding!
Thank you so much for your explanation it was wonderful. I am having one doubt we haven't coded to go up if we don't have left or right children, then how it goes up programaticaly
Hi Tammay. Thanks for the comment. You might want to check out my recursion playlist if that part is tripping you up. I think it clarifies your confusion. Cheers!
@@LucidProgramming Ok Thank you so much sir. I will go through that playlist first.
@@hackytech7494 Cheers!
Im trying to wrap my head around saving the recursive calls in two variables both named traversal in the preorder_print function.. I have spent quite a lot of time learning recursive functions recently (including your videos and others), and thought I understoodd them well for things like binary search and what not, but I still can not picture/map what is going on in my head with those calls in that function, I am getting extremely frustrated right now. Great channel btw, subscribed.
Recursion is one of those things that I think takes a while in wrapping ones head around. I've worked directly with Sam from Byte-by-Byte and he has some really fantastic material on recursion. Specifically, he provides a free e-book on recursion and dynamic programming that has really great visuals of which I haven't really seen elsewhere. I'd recommend giving that a look.
And, also, don't get discouraged. Learning these things takes time and practice. Thanks for the support and subscription.
Hello, great video. What if I wanted to create a nested list that represented the tree?
Thanks. I'm not sure how to answer the question. It's kind of like asking "how can I loop over the elements in the tree"? Well, you just use a for loop to do so.
what is the time complexity of your preorder recursive traversal function?
Just incase there's someone like me out there: I struggled forever to understand how this code and recursive data structures generally worked until I finally understood recursion. Al Sweigart has a great video on recursion but essentially recursive functions use a stack method to determine how the functions are called. the subsequent functions can't be called until the previous one has been removed from the stack. each number being appended to the string is that function being removed from the stack. So, counterintuitively, the string begins with one because it's the LAST METHOD to be cleared from the stack. All, the other methods cycle through the tree first, appending their value to the string. It sounds backwards, I know. I don't even know if I explained it well but you have to understand recursion to understand BST. Watch Sweigart's video.
Cool, thanks for sharing and thanks for watching!
Sir in each order_print functions how node gets back to head position when its left child or right child becomes None.
Through the recursive call of the function (if I understand the question correctly).
@@LucidProgramming Thanks for the great set of videos. My go to playlists.
On this one - a bit confused as to how the recursion is getting back to the right child. In the example, after reaching 4, the left child is none. So the recursive function should not enter the loop at all, so how is reading the right child (5)? I get the code intuitively, but am stuck when trying to create the stack line by line to understand the recursion well.
@@ajrvasu I think what might be insightful is to print out the elements as you make the recursive calls. That might make it a bit clearer as to what is happening. Hope that helps!
@@LucidProgramming Thanks for the suggestion.
@@ajrvasu No problem, cheers!
Hi- I think the code for post-order traversal might be incorrect.
Is it supposed to output: [4, 5, 2, 6, 7, 3, 1]? Saw a similar tree in this article with this as the output: www.geeksforgeeks.org/iterative-postorder-traversal-using-stack/
Hi Minali. You are absolutely correct. I checked to make sure the other traversals are correct, and they appear to be. As you say though, there seems to be something incorrect about the post-order traversal. I will be updating the code and also adding a note to the video to ensure others are aware of this. Thank you very much for pointing out my error, I sincerely appreciate this as it improves the quality of the content on this channel. Cheers, and thank you again.
No problem! Thanks for fixing it!
Ah actually, I believe my issue here was I was calling the "inorder" function inside of the "postorder" function. Another helpful commenter pointed this out and I have since fixed the code. D'oh! :) Thanks again for letting me know, and thanks for your help!
Thank you sir!
You're very welcome. Thank you for watching!
@@LucidProgramming I watched many videos on this subject, yours is spot on!
You're a great teacher, thanks again!
@@user-jc3gx2mj9u thank you for the kind words!
Thanks for thsi amazing content but I have one question in the print function where is tree coming from?
for example line 3 in the code below, there is no varible called tree so I don't understand why you are using? and instead can I use self.root.
Thanks
1.def print_tree(self, traversal_type):
2.if traversal_type == "pre_order":
3. return self.pre_order(tree.root, " ")
4.elif traversal_type == "in_order":
5. return self.in_order(tree.root, " ")
6.elif traversal_type == "post_order":
7.return self.post_order(tree.root, " ")
else:
print("Traversal" + str(traversal_type), " is not supported")
return False
Hi Noor. If you print the type of tree, this might cast light as to what this is referring to. For instance: print(type(tree)). Using tree.root or self.root have the same effect.
Thank you 🙏🙏🙏
I have a doubt, after a while adding nodes by writing tree.root.left.right.left bla bla becomes a cumbersome process, if you could suggest a better and easier way to add nodes than it would be a great help, thanks.
There must always be a way!
(17:57) Why in preorder_print (line 4) you passing tree.root? why not self.root? to me it's not clear how Class knows about it's instance?
Can you please explain?
Thank you
Thanks Sir, what is difference b/w class Node(object): and class Node:
No difference at all!
That's right. No difference. According to the link below: in Python 2.7 and older, you need to add the object inside parenthesis. In Python 3 and above, it doesn't make a difference.
introtopython.org/classes.html
@@amithpandit4043 kk tq :)
@19:30 i don't know if my doubt sounds silly but I want to know how it is moved upward when there is no left or right subtree
I didn't get that
please can you explain??
and thank you for this amazing video : )
Thanks, Nivi. I think you might benefit from checking out my recursion playlist as that might clear this up. Hope that helps!
@@LucidProgramming ohh thanks a lot , I'll check that
@@nivi8319 Cheers, let me know if you have questions!
explains it way better than my ds&a textbook lol
Cheers! If you enjoyed and benefited from my content, please consider liking the video and subscribing to the channel for more content like this. If you would like to support the content creation on this channel please consider unblocking ads when watching my videos as this is how I support my time to make content. I hope to be putting out more similar videos soon!
how are you able to access attributes from another class without creating an object for it or even using inheritance? plz explain
I think you might need to look into object oriented design on your own. I'm not going to give you a lecture on OOP principles in a RUclips comment.
best explanation on youtube!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Haha, cheers Spamdeep, I really appreciate that :). Happy coding, and thanks for the kind comment!
@@LucidProgramming actually the name is supamdeep not spamdeep
@@ravitanwar9537 Whoops. Must have been my autocorrect? Sorry about that. Thanks for catching, Ravi.
Hi, why when you define the node class or binary class it inherits from object class?
This is the way in which you define a class in Python.
For the output on 19:38 I received an output "Traversal type preorder is not supported." Is that the correct output? Am I missing something?
You should check your code against mine on GitHub.
Can you tell me what does "start.value" mean in the traversal methods? I am not that great in understanding classes. Thanks for the great video btw.
We are accessing the attribute "value" associated to the node object "start".
@@LucidProgramming Thanks a lot!
@@ritviks.8587 Cheers! If you enjoyed and benefited from my content, please consider liking the video and subscribing to the channel for more content like this. I hope to be putting out more similar videos soon!
Sir from where you draw trees in PPT.
Can you suggest any app
Shapes? If you can draw a circle and draw line, you can draw a tree.
Can you explain the recursion in pre-order traversal?
Got a whole playlist on recursion you can check out!
Thank you SO MUCH for the great tutorial, one of the best I've seen so far! I just found your channel and it is a real treasure! please keep up the great work!
I'm new to python :) so please forgive my silly question! in the helper function print_tree, you passed tree.root to the 3 printing functions, maybe I'm missing something here but shouldn't it be self.root instead of tree.root?
Thank you again!
Hi Toufi. First of all, welcome to the world of Python! :) The world of programming and computer science is a great one, and I hope you find it to be as pleasant as I do. Thanks very much for your kind words, I do indeed intend to continue producing more, and, hopefully progressively better content.
Thanks for your question. Indeed, that's a good one, and without knowing some of the subtler points of Python, this can seem a bit confusing. The parameter "self" is a convention used in Python to denote a class variable. We created a BST class, and any time we want to refer to a specific variable in that class, we can make use of the self keyword to do so.
Now, in the bottom part of the program, we create an instance of a BST object based on our class. The "self" variable is not accessible to us on this level as it is only accessible inside the class. For more information about these sorts of things, I'd recommend reading up a bit on Python classes. I think that should hopefully clear up any confusion.
Thanks again for the question and the kind words. Cheers!
@@LucidProgramming Yes but if we didn't instantiate BinaryTree class as 'tree' then it won't work. So 'self' should be the right approach here.
In the postorder_print function, I think you are calling the inorder_print function for the left and right nodes erroneously (copy/paste error).
Yep, you are indeed correct. Looks like I too hastily copied and pasted from the previous traversal. I'll make sure to update my code on Github to reflect the correct traversal algorithm. Nice catch, and thanks for watching!
Not a problem. Thank you for posting. All of this is pretty helpful and makes things more clear for the rest of us.
Any time Ahmer, happy to hear that these videos are helpful to others. Cheers and have a nice day!
Hello, I looked for traversals on your Github. Couldnt find it. Could you post a link or code correction? Thanks for taking the time to share you knowledge with the masses!
This is gold
Thanks, Sam! I'm glad that this video was useful to you. Thanks for the comment and for watching! Cheers.
For the functions being returned in the print_tree method, what difference would it make if self.root was used as arguments instead of tree.root? Doesn't tree.root limit the class to just that object?
Have you tried doing that?
@@LucidProgramming I did and now it works with different named trees.
@@redpred3502 Right, because "self" and "tree" both refer to an instance of the Tree class.
@@LucidProgramming and by using "self.root" can make the function works for an instance with any name other than the name "tree", right? thanks
@@ZhuYiting612 So long as you're referring to the object created in the class.
crystal clear
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If I'm not mistaken, depth of root node is 0. Root node is at level 0, not 1 :)
What are your vim settings/plugins? I like it
Good question! Much of my configuration is covered here: ruclips.net/video/vlb3qUiS2ZY/видео.html
Thank you.
Cheers! If you enjoyed and benefited from my content, please consider liking the video and subscribing to the channel for more content like this. If you would like to support the content creation on this channel please consider unblocking ads when watching my videos as this is how I support my time to make content. I hope to be putting out more similar videos soon!
Amazing content but one question, why I thought in Binary trees the left_child should be less than the root, in that case if the root is 1 then the left_child cannot be 2. May be I am wrong but I appreciate if clarify that, thanks again for the amazing content.
Thanks Noor. Regarding your comment, you're thinking of binary search trees. I have a whole playlist on them as well that you might enjoy. Cheers!
Thanks for reply, I am checking your page but cannot see the Binary Search Tree vidoes, I have seen Data structures which consists of 40 video and the last 7 deals with Trees, I am not sure if they are contained there. Thanks.
Hi Noor. There is a BST playlist. You can find it here: bit.ly/lp_bst. Cheers!
Thanks @@LucidProgramming
@@NoorAli-uh4uq No problem! If you like my content, I've been working on some projects during the past couple of months. If you would like to stay up-to-date, please consider subscribing to my mail list. Also, if you haven't already, please consider subscribing! I really appreciate the support. I hope that the content I provide there will enhance the videos on my RUclips page.
bit.ly/lp_email
Great stuff. I've been enjoying these videos so far. I'm a bit struggling to understand how recursion works in preorder_print. What's exactly happening when we hit a node with no left child, how the function is able to revert back to the previous node. I can't understand this part, can you please help me with that? Thanks.
I have a playlist on recursion that might be worth a look if that part if holding you up. Cheers!
I see several people struggle to understand how the recursive function moves up to previous node. In simple terms, we call the preorder_print function with the root (it is now stored in stack) which recursively calls the function with root.left, gets stored in stack (which also in return calls the function again with its left node, gets stored in stack) and all these functions are dumped from the stack when they return something. (when you call the recursive function with the root, it firsts call the function with its left node and when this returned something, the function moves to the next line, it prints its value and then calls the function with its right.
@@haciiiiiiii Thanks, very helpful !
inorder traversal result is supposed to be in sorted order . any ideas ?
Hi Ravi. Not necessarily for any binary tree, no. If you applied an inorder traversal to a binary search tree, then yes, the nodes would appear in order based on the ordering principle of the binary search tree. This is not necessarily true for a binary tree as it does not have this ordering principle imposed on it. Hope that makes sense, and thanks for your question.
I should also say that "in order" is with respect to how one reads the nodes of a binary tree when reading the nodes from left to right. Note that, of course if the data values of the nodes are not likewise ordered in such a fashion, an in order traversal will not yield an output in sequential order. Hope that clarifies things further. Cheers.
yeah you are right . that was binary search tree i was talking about .thanks for such an informative reply.
can you share your email id ?
Great, glad that cleared things up. You can find my contact in the "about" section of my channel. However, unless you intend to reach out regarding a tutoring or consulting session, I would encourage you to keep communication in the comment section so as to help other viewers who may be encountering similar problems or who have similar questions. Cheers!
I think there is a little error in the "print_tree()" function, it shouldn't be "tree.root" it should be "self.root" ! Plus the "postorder" error pointed out in some comments below!
Still, it is a great effort. Thanks.
Indeed, thank you for the comment!
I am on Pycharm and I am a beginner. I do not understand what you're doing at 18:15.
I can force-erase the consol but I can't write inside it.
Why you passing object with every class?is it mandatory in python3.x
No, it's not mandatory, but it makes it explicit if you have multiple classes in the future and want to use inheritance between them. You can take it out, and it won't make any difference. I just find the explicit nature to be a bit more clear, but you can decide in your own code whether that is the case or not.
I didn't get that in Inorder traversal like how from node A it's Going back to Node B?
You might want to check out my playlist on recursion if this part is confusing to you.
checking for None using a bool cast is also an anti-pattern. Use "if start is not None" instead.
Thnq sir I am facing a TypeError: Binary tree takes no argument .
Tell me about this
You might want to check your code against my GitHub code.