Largest Divisible Subset - Leetcode 368 - Python

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Problem Link: leetcode.com/problems/largest...
0:00 - Read the problem
0:18 - Memoization Explanation
8:53 - Coding Memoization
9:17 - Optimized Memoization Explanation
11:31 - Coding Optimized Memoization
15:40 - DP Explanation
18:57 - Coding DP
leetcode 368
#neetcode #leetcode #python

[@priyam86f]

hey man...after all this leetcoding and years of wait, finally got an interview today. the least I could do is thank you for all the help!

[@leeroymlg4692]

how'd it go?

[@priyam86f]

it was miserable.. was my first interview, i went way too nervous, basically based on an assignment where i had to build a REST API. the interviewer asked me to tweak the database design where i failed at. I would not consider myself a failure, instead i wrote a doc highlighting all the mistakes i made..and trust me there is a lot more to learn by yourself when you think from a calm mind.@@leeroymlg4692

[@shay2559]

probably failed it @@leeroymlg4692

[@jiangnan1909]

Hey Neetcode, big thanks for your algo videos! I just got a Facebook E4 offer after following you for a year. Your teachings made a huge difference in my algo rounds. Cheers!

[@NeetCodeIO]

Congrats you deserve it!! Glad your hard work paid off 💪

[@BookScreenTalk]

Was searching for your explanation for this question, and it ends up being uploaded after an hour 😊 I just want to take a few seconds and convey how useful your explanations are. Even though there are plenty of other videos explaining questions, I always end up looking for Neetcode's videos, because I know for a fact, it does not waste time, makes us understand the concept with minimal words. There are plenty of people with the knowledge of algorithms, but only a select few know how to teach them well. Thanks for everything that you do 🙏

[@razataggarwal7365]

Life saver, I tried it for a 2.5 hours and could not optimize brute force. I was not able to break my problem in terms of overlapping subproblems. Thanks for your efforts. Now I can save lot of time thinking in right direction. Keep it up 🙌🥳

[@olcaytugurkan9958]

Thank you for your consistency in daily problems!

[@user-kt4gp5fb8q]

This video was perfect, I always like the recursive approach as they are easy to come up, but bottom up is more efficient, you showed how to convert one to another, which is what I was looking for in every DP solution. Thanks! for that

[@aadil4236]

Great explanation! I got it in the first 3 minutes. Thanks!

[@MP-ny3ep]

Great explanation as always. Thank you so much.

[@lavanyam3224]

please include company logos in the video thumbnail like you did in your earlier videos.. it really helps to pick company specific questions

[@kareemadesola6522]

Fantastic solution. Intuitive as always particularly converting from recursion to tabulation I modified your solution to modify dp[i] only when dp[j] >= dp[I]; It made it more time and space efficient. Not in big O though class Solution: def largestDivisibleSubset(self, nums: List[int]) -> List[int]: nums.sort() n = len(nums) dp = [[val] for val in nums] res = [] for i in range(n - 1, -1, -1): for j in range(i + 1, n): if nums[j] % nums[i] == 0 and len(dp[j]) >= len(dp[i]): dp[i] = [nums[i]] + dp[j] res = dp[i] if len(dp[i]) > len(res) else res return res

[@adityapathak2509]

I have already solved this question, but I'm here to see your approach....

[@Marcox385]

When I'm lost at a problem and have to look in the "Related Topics" section, I'm always relieved to see "Sorting", however, and itch remains when thinking about a possible solution that doesn't use sort

[@woowenjun550]

I used topological sort (Kahn) because I couldn’t figure out the dp solution 😅 Basically we can visualise it as a directed acyclic graph where every edge (src, dest) means that dest % src == 0 and it represents a dependency. For [3, 17], since 17 cannot be divisible by 3, we can return the first element in the array. For [2, 4, 6], 4 and 6 are dependent on 2, but 6 is not dependent on 4. This means that when we build a predecessors map, we have { 6:2, 4:2 } and we just have to travel back from each element to its parent and get the length of the path. For [2, 4, 8], 4 is dependent on 2 and 8 is dependent on 4. This means that we cannot process 8 until we process 4 and we cannot process 4 until we process 2. The predecessor map will be { 4:2, 8:4 }. Time complexity is O(n^2) too but I think space complexity wise might be O(n^2) as it might be a dense graph

[@mohamedanas8493]

iam studying 4th sem in computer department when i saw this video, i fell like i don't know anything, can i able to think or solve problem, like this in future can someone replay

[@bulioh]

Great video. One thing I noticed is once you've sorted the input, this becomes super similar to leetcode.com/problems/longest-increasing-subsequence, except you're caching arrays instead of integers

[@user-tg6jl2nz7o]

Actually, Python has a decorated version for memoizing DFS, which I think is more convenient: @cache def DFS(*args): do something here; return res This is equal to: cache = {} def DFS(*args): if args in cache: return cache[args] do something here; cache[args] = res return res

[@zaki_1337]

he uses that sometimes

[@NeetCodeIO]

Personally, if I was interviewing someone who used the decorator, I would ask them to actually write out the caching logic

[@zaki_1337]

@@NeetCodeIO that's pretty easy to do.

[@no_mnom]

Awesome explanation but it feels like a pretty simple one.

[@sankalppatil2994]

Great explanation as always! I finally solved this on my own basically coming up with the same dp approach but it took me around 30 minutes. Any tips/tricks for getting faster? I'm not sure what the upper limit is for interviews but I'm assuming ~30 minutes is too much time.

[@user-tg6jl2nz7o]

BTW, slightly modified version of the last iteration, which makes the time complexity more efficient: nums.sort() dp = [[] for n in nums] ## intialize empty list for i in reversed(range(len(nums))): k = i # # initialize trace index as i for j in range(i+1, len(nums)): if nums[j] % nums[i] == 0 and len(dp[j]) > len(dp[k]): k = j ## update index k dp[i].extend(dp[k]) ## Add the longest sequence with index equal to k dp[i].append(nums[i]) ## finally append the nums[i] return max(dp, key = lambda x: len(x)) ## return the longest sequence

[@bedruomer8777]

return max(dp, key=lambda x: len(x)) is more concise, thanks for the detailed explanation!

[@vidallera8869]

you can do key=len :)

[@satyamjha68]

Love your Videos NeetCode!! Solved it using Bottom Up approach !! And ya do you plan to make some more system design videos where you go through a blog/article just as you did for Facebook Priority Message Queues?

[@NeetCodeIO]

Yeah, hopefully pretty soon!

[@coffeebytes3257]

😭 anyone have any additional resources to understand DP / build intuition behind it? Currently working my way from NC 1-D problems, just got to 2nd problem

[@JAson-ps2ug]

i think this problem is more complicated with top-down approach, the botton down approach is much clearer and easier to understand, should start with bottom-up

[@NeetCodeIO]

Yeah I agree top down is more complex in this case

[@sauravchandra10]

is the brute force getting accepted, mine is showing empty list as an answer for every case

[@devduttabiswas2204]

Hey man, thanks for the videos. I actually got the neetcode pro today but I am not able to see the videos as an error message pops up saying that "Sorry, Because of its privacy settings, this video cannot be played here." Can you please take a look into it?? Thanks

[@NeetCodeIO]

Most likely you have an ad blocker or VPN/proxy that is interfering with it. Feel free to email me if the issue persists.

[@devduttabiswas2204]

@@NeetCodeIO Thanks man, I figuered out the issue because of you and fixed it!!! Appreciate it!!!

[@tizekodnoon8305]

Neato, Brain fried!!!

[@pastori2672]

i gave up watched 50 seconds of the video and realized the mistake 😅 im actually kinda proud about this one

[@pastori2672]

it was the tabulation solution i came up with so thats why

[@naamnhibataunga5897]

can someone explain me for the input=[4,8,240,10] expected output-[4,8,240] my ans- [4,8,240,10] how the expected output doesn't contain 10??

[@mhdsbk]

Here 8 and 4 are not factor of 10. so if we are including 10 the answer will become [10,240]. LONG ANSWER - If you pick 4, you can pick only multiples of 4. 4, 8, 12, 16..... 240,244.... etc - see we cannot pick 10. - then we come to 8, we can pick it if prev was 4 since 4 is factor of 8. - if we pick 8 moving forward, we can only pick 8, 16, 24 ... 240, 248, ... - so we can pick 240 - But if we want to pick 10, we can then pick multipples of 10 which are 10, 20, 30....240.. in our case the answer will be [10, 240]

[@anonymoussloth6687]

Can't we just do this like the LIS problem?

[@chien-yuyeh9386]

First!

[@zeroanims4113]

can someone explain why this results to TLE? i have the same code as the memoized python solution in the video: vector rcMemoWay(int i, int d, vector& nums, vector& memo){ if (i == nums.size()) return {}; if (memo[i].find(d) != memo[i].end()) return memo[i][d]; vector ans1 = rcMemoWay(i+1, d, nums, memo); if (nums[i] % d == 0){ vector ans2 = rcMemoWay(i+1, nums[i], nums, memo); ans2.push_back(nums[i]); return memo[i][d] = (ans1.size() > ans2.size() ? ans1 : ans2); } return memo[i][d] = ans1; }

[@shaco6630]

It seems this solution causes TLE in different language except Python. I got TLE in Kotlin too.

[@user-tg6jl2nz7o]

I copy your c++ code, and submit on leetcode, and passed 47 cases, and other 2 cases failed because of Time Limit Exceeded, then I chose one of the failed case whose length of nums is 1000 to test, if use your c++ code to run, the running time will took about 1300ms on leetcode platform, however if using the python code by Neetcode to test the same failed case, only took about 570ms, almost half time of c++ code. I think the possible reason, is the dict used in python is more efficient than the unordered_map used in c++. PS: c++ code: class Solution { public: vector largestDivisibleSubset(vector& nums) { sort(nums.begin(),nums.end()); unordered_map tmp_map; vector memo(nums.size(), tmp_map); return rcMemoWay(0, 1, nums, memo); } vector rcMemoWay(int i, int d, vector& nums, vector& memo){ if (i == nums.size()) return {}; if (memo[i].find(d) != memo[i].end()) return memo[i][d]; vector ans1 = rcMemoWay(i+1, d, nums, memo); if (nums[i] % d == 0){ vector ans2 = rcMemoWay(i+1, nums[i], nums, memo); ans2.push_back(nums[i]); return memo[i][d] = (ans1.size() > ans2.size() ? ans1 : ans2); } return memo[i][d] = ans1; } }; Python code: class Solution: def largestDivisibleSubset(self, nums: List[int]) -> List[int]: nums.sort() @cache def dfs(i, prev): if i == len(nums): return [] res = dfs(i+1, prev) if nums[i] % prev == 0: tmp = [nums[i]] + dfs(i+1, nums[i]) if len(tmp) > len(res): res = tmp return res return dfs(0, 1) Test case: 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[@birdbeakbeardneck3617]

dict?

[@xingyuxiang1637]

From @cache to cache = {}. Learn a bit about hashable. Then, from cache = {} to bottom-up dp. The bottom-up DP is to spell out how the cache is cached. Similar to cache = {} spells out @cache. The next level is the medium backtracking problem. Not only does it use two caches, but it also mixes linear codes and nonlinear codes. The hard level again uses a magic number such as 8000 for the LRU cache. Do not worry about hard levels until they get intuitive and fun. Get more about medium matrix problems by using NumPy. The path should be from easy to medium. Then, from medium to hard.

[@rajsuriyang3427]

I thought this was a backtracking problem.

[@deathbombs]

Hard question cause I hate math. Factors giving migraines

[@philipr3011]

👉 "Promo sm"