Flatten Binary Tree to Linked List - Leetcode 114 - Python

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Problem Link: leetcode.com/problems/flatten...
0:00 - Read the problem
1:20 - Drawing Explanation
6:38 - Coding Explanation
leetcode 114
This question was identified as a facebook interview question from here: github.com/xizhengszhang/Leet...
#facebook #interview #python
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26 июл 2024

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Комментарии : 57

@NeetCode 2 года назад

🌲 TREE PLAYLIST: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-OnSn2XEQ4MY.html

@srinadhp 2 года назад

You make complicated problems appear so simple and make us guilty on why we could not think of that solution!! Great explanation and code as usual! Thank you!!

@jonaskhanwald566 2 года назад

Unbelievable sir. How are you still solving these problems.

@watchlistsclips3196 2 года назад

@@jonaskhanwald566 Heyy how is martha???

@shivampatel8928 2 года назад

One of the best programming channels on RU-vid.. Hands down please keep doing this

@yaadata 2 года назад

I just did this the other day [Wanted to try one that wasn't on your playlist yet] ! Man you are fast though, it took me 15 minutes to even come up with the appropriate strategy. Keep it up. My favorite channel

@akhilrajnambiar2080 2 года назад

Blessed are we to have such a channel, for helping us. Please do keep doing the amazing work!!

@aryanmaniyar3475 9 месяцев назад

I was struggling so hard, because out of all the videos I watched for this question, everyone seemed to use a very vague recursive logic without much intuition of the logic behind it. The way you told to write the recursive calls, the way you handled edge cases, I was able to understand it so clearly, that I was able to code your solution without seeing, as well as modify my own code which gave wrong answer while submitting three times, to work correctly for the fourth time!!! Thank you so very much :)

@fanifyeyev 2 года назад

best channel, anytime I need good explanation, NeetCode is very helpful

@susquon 10 месяцев назад

This was tough for me to solve on my own. It makes so much sense now thank you!

@Arizala213 2 года назад

Very helpful, thank you!

@ganapathinaik5446 2 года назад

Very clear explanation. Thank you :)

@muhammadmustafa3158 2 года назад

Best tutorials on youtube by far !!!!

@Arkham_nine 2 года назад

Thanks! What about "Follow up: Can you flatten the tree in-place (with O(1) extra space)?"

@spikygoldfish Месяц назад

The solution in this video uses O(1) memory, so the answer is "yes"

@jesussepulveda9992 2 года назад

Thank you all your videos are very usefull

@osiris1102 2 года назад

I am your big fan, but please make more videos on dynamic programming as it's one of the most confusing topic. Thanks.

@willdazns 2 года назад

Thanks!

@henrydi800 2 года назад

what are the values that returned from leftTrail=dfs(root.left) and rightTrial=dfs(root.right)?

@sanandmath4127 3 месяца назад

Excellent explanation! I have a follow up question on this. Given the flattened LinkedList as arrived in this solution, can we traverse this flattened LinkedList and reconstruct the original Binary Tree? Thanks

@ayoubalem865 2 года назад

keep going , everyday i wait for you to post a video and go try it before returning to watch your explanation, and sometimes my solution looks the same like yours : def flatten(self, root: Optional[TreeNode]) -> None: """ Do not return anything, modify root in-place instead. """ self.dfs(root) def dfs(self, root): if not root: return if not (root.left or root.right): print(root.val) return root r, l = self.dfs(root.right), self.dfs(root.left) if root.left: l.right = root.right root.right = root.left root.left = None last = r if r else l return last

@sirmidor Год назад

Keeping track of the order while recursing seemed difficult to me, so I solved it using a deque instead, using the intuition that preorder traversal is essentially popping from the left and appending potential child nodes back to the left of the deque: class Solution: def flatten(self, root: Optional[TreeNode]) -> None: """ Do not return anything, modify root in-place instead. """ q = deque() q.append(root) q.append(None) while q[0] is not None: node = q.popleft() if node.right: q.appendleft(node.right) if node.left: q.appendleft(node.left) node.left = None node.right = q[0] return root

@zehuazhou3390 2 года назад

Follow up: Can you flatten the tree in-place (with O(1) extra space)? I saw that follow up. If I use O(1) extra space then I have to move the right subtree under the right most position of the left subtree to avoid extra O(h) storage. But if I do that, the running time would be O(nlog(n)). Anyone has any better ideas?

@david-nb5ug 5 месяцев назад

+1 the follow up is confusing :(

@closingtheloop2593 3 месяца назад

Brute force is almost same complexity. DFS and add node pointer to list. Then iterate list and point list[n].right = list[n+1] and list[n].left = NULL. O(n) time and space. Simple af.

@tomislam 10 месяцев назад

Timestamp 13:05 - 13:06: Discord notification alert. :)

@jonaskhanwald566 2 года назад

Beautiful

@utkarshsharma1550 2 года назад

Your code is returning the pointer to Node: 6 if we take the example given in the question. Shouldn't it return the pointer to Node: 1. In short why are we returning the pointer to the tail?

@sidazhong2019 10 месяцев назад

"Return rightTail or leftTail or root" is too hard to understand. It's not simply like the question wants us to return the tail.

@utkarshsinh1919 Год назад

Why we have to return the list tail?

@ShivangiSingh-wc3gk 2 года назад

I have a few questions: 1) when doing the swapping: I thought of something like this root.left= None leftTail.right = rightTail root.right = leftTail. Why are we using the root and not the tails.

@thecuriousengineer 2 года назад

Because at each node we simply get a tail value from left branch. The tail value order is (rightTail, leftTail or root). At root (1) leftTail is (4) rightTail is (6) leftTail.right = root.right 4 -> 5 root.right = root.left 1 -> 2 root.left = None 1 / \ NULL 2 \ 4 \ 5 Try solving it on paper and you'll see it :)

@sidazhong2019 10 месяцев назад

leftTail.right = rightTail is wrong. rightTail is just a single node. root.right is the sub tree. root,right is a tree, rightTail is a node

@prafulparashar9849 2 года назад

Great explanation: Here i tried this question by traversing the right most subtree first -- I found it a little more easier to understand. ``` class Solution: def flatten(self, root: Optional[TreeNode]) -> None: """ Do not return anything, modify root in-place instead. """ self.temp = None def helper(root): if not root: return helper(root.right) helper(root.left) root.right = self.temp root.left = None self.temp = root helper(root) ```

@JyotiprakashMandal-bp8ng 3 месяца назад

Love you

@rajendarsingh9333 2 года назад

please make video of sudoku solver

@yaadata 2 года назад

For the sudoku solver just use 3 hashsets (One for the row, one for the column, one for the section of the board). The tricky part is for the section is a math equation to turn row and column number to section number (think section=m*row + column). Traverse the game board and check that no duplicates are in your hashsets. Time O(m*n), Space Complexity O(m*n) where m = number of rows, n=number of columns

@ruthviks 9 месяцев назад

Problem before seeing Neetcode: Wth is this?! Problem after seeing Neetcode: Oh. Such a simple problem!

@jkk23-g7c 9 месяцев назад

This question is one of those that after watching the video 3 times, I'm still confused

@tabmax22 Год назад

no need for a helper function, can just write the same logic inside flatten :)

@anotheraleks Год назад

he said that the only reason for helper was that the flatten func is hinted to return None

@saisumanth7389 4 месяца назад

But isn't it post order traversal?

@numberonep5404 2 года назад

A somewhat different solution: def flatten(self, root: Optional[TreeNode]) -> None: if root: # We keep track of the pointers to the left and the right subtrees left = root.left right = root.right # We cut the link between the root and the left and right subtrees for independance root.left = None root.right = None # We flatten each subtree self.flatten(left) self.flatten(right) # We reattach the first part to the root, meaning the left part since it is a pre-order thingy root.right = left # We also need to reattach the flattened right subtree to the end of left subtree prev, current = root, left while current is not None: prev = current current = current.right prev.right = right

@light2929 2 года назад

Isn't it O(n*h) solution 🤔

@numberonep5404 2 года назад

@@light2929 nope

@starsky9832 6 месяцев назад

This is very clever, thanks

@edwardteach2 2 года назад

U a God. I had a different condition in the if root.left condition. The code in the video doesn't work anymore. class Solution(object): def flatten(self, root): """ :type root: TreeNode :rtype: None Do not return anything, modify root in-place instead. """ def dfs(root): if not root: return root left_node = dfs(root.left) right_node = dfs(root.right) if root.left: root.left = None root.right = left_node while left_node.right: left_node = left_node.right left_node.right = right_node return root dfs(root)

@ShivangiSingh-wc3gk 2 года назад

Does this work?

@DJSTEVE42 2 года назад

You're right, thanks for the updated solution, it works

@KylianMbappe07303 Год назад

could oyu explain it again

@danieleboch3224 2 года назад

what's "next" attribute? we do not have such in the TreeNode class

@anonymous-404 2 года назад

watch it again!

@CANIHAZURDREAMSPLS Год назад

discord msg

@aayushpagare9366 8 месяцев назад

is neetcode real ?

@codetrooper9279 2 месяца назад

Explanation was BULLSHIT

@RahulJain-ye4gz Месяц назад

""" 1 / \ 2 5 / \ \ 3 4 6 Step 1: Flatten the left subtree of 1 Call: dfs(1) Call: dfs(2) Call: dfs(3) root.left and root.right are None Return 3 (last node in the left subtree of 2) Call: dfs(4) root.left and root.right are None Return 4 (last node in the right subtree of 2) leftTail = 3, rightTail = 4 Set 3.right = 4 Set 2.right = 3, 2.left = None Return 4 (last node in the flattened subtree of 2) Step 2: Flatten the right subtree of 1 Call: dfs(5) Call: dfs(None) Return None Call: dfs(6) root.left and root.right are None Return 6 (last node in the right subtree of 5) leftTail = None, rightTail = 6 Return 6 (last node in the flattened subtree of 5) Step 3: Combine left and right subtrees of 1 leftTail = 4, rightTail = 6 Set 4.right = 5 Set 1.right = 2, 1.left = None Return 6 (last node in the flattened tree) """