Winning LeetCode Weekly Contest 182 + Explanations

1st! Explanations start at 16:10. If you have any questions, please ask them in the comments below, so that I can improve my explanations as well!
I would appreciate it if you liked this video and subscribed!
Website: williamlin.io/
Instagram: / tmwilliamlin168
Twitter: / tmwilliamlin168

[@FisherCoder]

00:31 Started 1st problem 01:04 Started 2nd problem 02:22 Started 3rd problem 05:36 Started 4th problem 15:20 ALL ACCEPTED! 16:10 Started explanation

[@tmwilliamlin168]

Wait I should totally pin this, I need to figure out how

[@manavdubey9581]

Me: reading 1st problem William: Starting 2nd problem Me: Shocked pikachu

[@noenting]

@@tmwilliamlin168 Hey, just started learing a little. Any CP guys that use Python?

[@JayPatel-jn7cu]

@@noenting you'll switch to c++ later

[@clipit4503]

@@noenting Yeah python is very good for starting with cp switch to c++ later in when you confindent btw learning cpp is gonna be easier if you already know a language like py

[@nodirhasan9843]

you uploaded the video before I even finish the contest. hatts off

[@rakkaalhazimi3672]

Lol what a speed

[@kharis6770]

he: make typing sounds auto cc: [Applause]

[@lerdi]

When the keyboard said "clickclickclickclickclick" I really felt that..

[@afeinhandler]

Would love a video on the resources that were helpful in your CP journey. You rock.

[@aleks99994]

dude you are so talented... I love your videos, wish you the best !

[@MattYang]

hahaha I saw you won and was wondering when this vid would come out, nice work! edit: For underground system, don't need to erase key, and don't need to initialize maps

[@atypicalambience3487]

I love that you just reversed the string instead of rewriting the loop.

[@ShashiBhushan2k13]

Thanks for the great explanation William. I was stuck on last problem for couple of hours now.

[@kushaalrana]

Thanks for the explanation also bro much needed it love the way you code🔥👌

[@boskomarinkovic5532]

I know nothing about code but this seems legit

[@stealth3002]

I've been coding for years and still have little to no idea what this guy is doing.

[@hihello7979]

He does it so fast I can’t understand what he is doing until I pause the video lol

[@JoaoVictor-lq5ew]

I love your videos man, so good

[@lander2024]

this guy is a legend

[@enertion6923]

man i love your channel its so satisfying althought i dont understand a thing (i know another Lua so yea)

[@yacinmessaoud186]

i started sweating just by watching ! simply amazing !

[@stonecoldcold2941]

Hey man , really a great job in explaining. Do this always :))

[@kartaLaLa]

Notification squad for tmw~

[@atifhu]

william literally code in 2x and speaks in 0.75x, what a guy

[@rikohermansyah]

i was like "WTF is this?!!!" i graduate from informatics degree but i hate coding so freaking much, but after see this kind of bullsh*t i think i want to start learn to code sometime someday or some year maybe. you and cs dojo are freaking awesome.. i'll subscribe your channel (with the bell on) after submiting this comment.

[@johnhammer8668]

The first problem says if there are multiple values, the largest to be returned and in the solution where is it implemented ?

[@halfblood_slayer5989]

Hey thanks man. Please do more haha. Bored in quarantine

[@fatihahshamsuru1370]

This guy should be on a Netflix series. It's a real life hacking story.

[@yinghanma1314]

the last question is hard af i spent a whole hour on that one and couldn't get passed

[@akashgupta13]

That's why Leetcode categorised it as *Hard* 😏

[@thegodfather7994]

@@akashgupta13 most hard problems are doable. This one had CP vibes in it

[@yinghanma1314]

@@thegodfather7994 exactly, even after reading some solutions i still didn't get it

[@jimmorrisshen]

@@akashgupta13 It is hard than usual.

[@siddharthabiswas2147]

it does'nt require any ingenuity though

[@praveenchiliveri2049]

Great Video! Learnt a lot from it .

[@gamerlion472]

He's the Flash.

[@siplinayishoto]

Congratulations on the win! Also, what kind of keyboard do you use, because it does not sound like one of the high tech keyboards. It sounds more like an old keyboard.

[@utsavsingh899]

Can you make a video on your gvim setup, the plugins you use and other modifications that you have made?

[@fintech4220]

Yo I did this one also. My first one. I got slapped lol

[@aasmohammad2651]

this man is faster than a computer even a computer can not solve at this speed

[@ahmedouyahya]

Hello William. I always wonder why almost all competitive programmers use C++. And also it will be very helpful if you make a video on how leetcode contest work. Thank you so much.

[@uo5412]

Awesome as always

[@satyabratanaik8112]

Hey tmw, I really like your speed. Btw, which version of mingw are you using? I am using mingw-w64 but mine seems slower than yours

[@tmwilliamlin168]

I think GCC 5.1? It might be a difference in computer speed

[@shubhammanu3916]

Hey William, can you please tell me that for solving a DP problem which approach should I learn? Recursive or iterative??

[@TheWisp1]

In Q3 you don't need to initialize the maps. They are just default initialized.

[@user-gh8kl8ff3v]

How long have you been coding and where did you learn all this magic? Very impressive!

[@fridayknight1387]

In the first question , were we supposed to operate on sorted arrays or unsorted arrays ? Please reply ASAP .

[@tonymok535]

Why youtube algorithm brings me here recently haha

[@ajaib1313]

Need to use slow motion to see his actions in beginning😍💚

[@vivekgupta4051]

gonna download that theme for chrome

[@dkssud5630]

What condition tilde i (~i) is in the for loop?? You used it in solve function for the last question. Thank you.

[@jimmyliaouwu]

www.cplusplus.com/doc/tutorial/operators/ ~ operator is unary complement (bit inversion) ~3 is -4 (true), ~2 is -3(true), ~1 is -2(true), ~0 is -1(true), ~-1 is 0(false), ~-2 is 1(true), so for (int i = n - 1; ~i; --i) equal for (int i = n - 1; i >=0; --i) It's faster for typing.

[@dkssud5630]

@@jimmyliaouwu Thank you so much!

[@wudong5422]

awesome man!!

[@siddharthpanigrahi3855]

hey William, did you ever do a problem like problem 4 or did idea just popped in your mind? And can you suggest some resources to study to improve my problem solving skills.

[@tmwilliamlin168]

Yes, two very similar problems appeared in many forms 1. Count number of strings < s with digit DP 2. Count number of strings not containing a given string

[@sarmaakondi485]

Hi Lin, your videos are awesome. Can you recommend any books or tutorials to become competitive programmer?

[@tmwilliamlin168]

I will

[@sarmaakondi485]

@@tmwilliamlin168 looking forward

[@shalomalexander7811]

2 Years have passed!

[@jimmorrisshen]

This is great and with clear explanation. Thanks a lot.

[@k4i976]

stay awesome!

[@lawrenceliu401]

How would you recommend getting from knowing the basics of programming to starting to understand competition problems? Any competition or place to start on?

[@xegativ]

Lawrence Liu ^

[@halfblood_slayer5989]

just jump into it XD everything in the world is practice if not talent haha edit : also subscribe and hit the bell button, maybe you'll have better luck

[@tmwilliamlin168]

You want to start doing basic problems & learn basic algorithms. After that, it's just practice. I will make videos with more info~

[@CuriousCI]

I can't do leetcode's contests: it's 4:30 AM here in Italy when they start!

[@tmwilliamlin168]

That's sad, at least you can do the biweeklies

[@filipposcaramuzza2953]

Eh a chi lo dici maledizione

[@supermig2287]

That's a juicy fruit for FANG, right here!

[@TechnoPrefix]

You are awesome

[@akashgupta13]

3rd Problem can be done in another way (probably simpler) - leetcode.com/problems/design-underground-system/discuss/554941/Java-Simple-HashMap-Solution-Clean-code Maintain 2 maps - one for checkin and one for checkout in the following way Map < Station , Map > Calculate average by iterating through checkIn map of station Add sum and count each id present in checkin[starttSation] and checkout[endStation], find average as sum / count.

[@psw1]

Did you pass the problem with this approach? mine got TLE. Please share a link to your code if possible.

[@akashgupta13]

@@psw1 Accepted - leetcode.com/problems/design-underground-system/discuss/554941/Java-Simple-HashMap-Solution-Clean-code Submission - leetcode.com/contest/weekly-contest-182/submissions/detail/316962918/

[@sadn1ck]

how would you recommend a beginner and someone who's bad at problem solving to get started with Competitive Coding and/or DSA

[@tmwilliamlin168]

Learn basic algos first then just keep practicing. I will make videos explaining more.

[@chrischen4584]

太神啦威廉 偶像

[@jkim5118]

Man, you are amazing. What is your goal? Where do you want to go to college and where do you want to work eventually?

[@tmwilliamlin168]

I'm going to MIT and I have no idea what I will do except that I will continue programming

[@jkim5118]

@@tmwilliamlin168 Amazing! I am pretty sure you can get into any company u want. Good luck man.

[@siddharthabiswas2147]

implemented the same solution in python for Q4 gut tle fr the last case

[@VivekShah98]

I solved the last problem in the contest but what would be the approach if there were multiple evil strings in the question, and we are asked to find the number of strings lexiographically less than given string and not containing all the evil strings as substrings?

[@tmwilliamlin168]

I think we will add more dimensions to the DP

[@samueldeng2336]

@@tmwilliamlin168 Cool, never thought of it that way (the more complicated the problem is, the more dimensions you add). Nice work

[@yunjiehong4649]

why dp[n][0][1] = 1? I do not reverse e, and stuck with the base case.

[@thl_leo]

Is there a reason why you put ++i instead of i++ in the for loops?

[@user-he2zw2os7k]

Very clearly

[@ardisujaya]

Could you give me advice on how to learn programming? I find it amazing when you type programs very quickly

[@TBasianeyes]

grind leetcode

[@saikalyaninduri7667]

Can you tell an optimal solution for second question ?

[@vamsia5543]

great video Bro, could you please provide a good strategy to become a good competitive programmer just like you :) . Thanks in advance.

[@DarkNix741]

Hey William, do you have any tips on typing that quickly? What is your WPM? Im on 120 and this looks 3 times faster than I code...lol Excellent video as usual!

[@tmwilliamlin168]

When I type normal sentences my WPM is 100-130, however, I code a lot, so that could explain the difference.

[@mehulsethi5886]

which IDE do you use for coding c++..i have not been able to find a good one lately..Help Me ..

[@tmwilliamlin168]

I use GVim, but it has a steep learning curve

[@realgeniusxxhackerxx4473]

Wait for the first problem how are u able to find the largest Lucky number like 1 ,2,2,3,3,3 ur code would return 1 and not 3

[@davudsafarli5356]

well, He don't return *and* if it meets condition inside if. He loops through all pairs inside map. I think using rbegin would be more efficient

[@JewelsPVP]

Subbed! Are you working in the computer science field in the future?

[@tmwilliamlin168]

Most likely

[@ervinzhang6387]

Awesome

[@lalom8245]

Yo this guy is sick

[@praneetsingh9695]

How did you learnt to implement your code so fast and win all the competitions????? any guide?

[@tmwilliamlin168]

In short, doing a lot of practice. I will post more guiding videos soon ~

[@praneetsingh9695]

@@tmwilliamlin168 how long it took you from 0 to hero?? from scratch?? can you upload a video telling us how much time did you spent on data structures and algorithms?

[@dezhongerzhang6130]

the last question i get passed theought dight dp method.

[@jimmorrisshen]

Cool. It means that you are top 76. I think you mean Digit DP. www.geeksforgeeks.org/digit-dp-introduction/

[@Sulerhy]

Your fast coding made my computer restart

[@CSERohitKartik]

Hey William, How did you learn Ds/Algo?

[@tmwilliamlin168]

I will teach soon

[@markliu22]

May I ask what keyboard you use?

[@jimmorrisshen]

This is great. I translated it to python and it works. Thanks a lot. BTW, I found some other people are using KMP to solve this problem. It seems KMP based are slightly faster. Could you please help us explore a little bit about the difference of using KMP and not using KMP? Thanks a lot.

[@tmwilliamlin168]

KMP is simply faster for calculating tr[][]

[@jimmorrisshen]

@@tmwilliamlin168 Thanks a lot.

[@adityashrivastava8733]

Can you please make the same kind of video for yesterday's atcoder beginner contest 160?

[@tmwilliamlin168]

I didn't have time, so I posted solutions on discord.gg/AneA5wg

[@adityashrivastava8733]

@@tmwilliamlin168 i accepted the invite but don't see any solutions on any of its threads. Can you please help it?

[@zachleach7118]

can you do a typeracer video

[@tianweiliu2555]

To learn the solution for the last problem.

[@rohitdighe852]

In question 1, it asks to return the largest luckynum if there are more than one, you didnt account for that and yet the solution was accepted. How so?

[@rohitdighe852]

Maps in c++ are sorted during creation.

[@rohitdighe852]

sorry i use python

[@rui133]

nice

[@shashanktiwari4442]

What would be the solution for problem B if N was large??

I think William strategy is good because he tries to write the faster answer based on the input constraints so that do not waste time on it because at the end it is a contest. For sure he knows how to make it with a better time complexity.

[@jamespaz4103]

I thought using nested loops is a bad idea, due to time complexity. But you do a pretty good job.

[@atifhu]

because constrains for n was only 200

[@youssefbennoursahli917]

what is your code editor ?

[@hemavardhansankati1161]

bro what keyboard do you use?

[@skipper2795]

How hard did you study for math?

[@XSTEVEN722X]

Do you have any tips on how to get started with competitive programming? I have my fair share of problems but I’m no where near your level! Also any tips with learning how to type? I’m currently learning how to touch type but I’m too slow! Great explanation btw!

[@tmwilliamlin168]

I'll make videos on starting CP soon~ Your touch typing will become better with practice.

[@akashverma5756]

TIme limit on leetcode contest is bit relaxed

[@shariqshaikh5715]

Please explain all the problems you have solved in your videos also

[@sanchitkumawat3803]

how to become better in solving dp questions?

[@tmwilliamlin168]

Do a lot of practice solving dp questions

[@dulanwaduge]

how did you start programming? did you learn from college??

[@tmwilliamlin168]

I learned from books. I have not graduated high school yet.

[@ajourney179]

What is the difference between you and gennady korotkevich.

[@stoicescueric8654]

What app do you use for coding?

[@tmwilliamlin168]

GVIM

[@mohamedSayed-lr4xs]

what is the type of your keyboard ?

[@tmwilliamlin168]

It's a mechanical keyboard

[@erikjanssen2940]

GoOgLe Is LoOkInG fOr YoU

[@Mohammedaatif90]

i need a month to understand the last problem.

[@zuowang5481]

how is 4th a interview question

[@tmwilliamlin168]

Maybe from Google?

[@yashkumar8472]

Told u. ..he is GOD

[@nith9009]

Hi man , the first question you could do it easier like this : for loop to iterate the array , if condition to compare the an incrementing variable with the current value of the array in java it would look like this : int ans = -1; int k =0; for (int i = 0 ; i

[@ilies-vip8978]

Can you explain it in french ?

[@erichuang5971]

How was usaco

[@tmwilliamlin168]

The contest hasn't ended yet, but you will know soon :)

[@erichuang5971]

Good luck at IOI this year 😛

[@pradyothcherry3426]

That Typing sound made me to close the video..!

[@obsidianskin9502]

what up

[@halfblood_slayer5989]

Playing battle royale games XD