Round 1006 (Div 3) problems A-F video solutions here: https://www.youtube.com/watch?v=3vsArf1QnwM

I know it is a few weeks after the contest, I meant to get this video out earlier, apologies for the delay. I figured better late than never, so I posted it anyways. I hope you enjoy the video.

I know there is another solution video as well (see streams for the video). That video and the editorial are great resources for this contest. I just thought I'd share my perspective as to how I approached the problems when I saw them for the first time and the steps I went through to arrive at the answer.

Contest strategy is a very important part of most OI contests (USACO, IOI, and similar contests for other countries).

The goal of good contest strategy is so that after the contest, you don't get the feeling that "Man, I could have gotten that subtask or that problem if I just had more time". That is the worst feeling ever, because you did the preparation, and you had the skills, but your skills just didn't convert to points in contest.

With good contest strategy, all the points that you are able to get, you get in contest. While I don't get this every single time, I get it about 90% of the time I compete now, which is a vast improvement from missing "easier" points in almost every contest a few years ago.

Here's a video I filmed detailing how you can improve your contest strategy in a week: https://youtu.be/9Fn1-0jfVik.

I put together an in depth video on how I used friendly competition to accelerate my progress, along with a few ways you can incorporate it into your programming journey. Hopefully you find this helpful, if you are interested you can check the video out here: https://www.youtube.com/watch?v=bgpaXDAOdBk

Yibo and I put together an in depth video on what procrastination in cp looks like and tips on how to avoid procrastinating. You can check the video out here: https://www.youtube.com/watch?v=yDjCh1i_LnQ

I made video solutions to all the problems (A-F) of Round #628 which you can find in this playlist here or using the links below.

Problems A and B
Problem C
Problem D
Problem E
Problem F

As a side note, with all my extra time from staying home, I'm coaching people who spend 6-10+ hours a week coding with targeted problems to improve rating. If you are interested, fill out this form and I'll reach out.

Hi CodeForces!

I made a tutorial on hashing which you can find it here.

Additionally, I am in the process of creating a new course. You can find details about it on the bottom of this page.

I recently streamed about persistence, specifically, persistent arrays, persistent segment trees, and persistent union-find. You can check out the stream here.

Hi CodeForces!

Last week I streamed about LCA (least common ancestor), Segment trees, and Heavy Light Decomposition. You can check out the stream here. If you already know one or more of those concepts, you can jump to the other concepts.

Hi CodeForces,

The week 6/7 stream was about data structures like Fenwick trees and segment trees. Note that you need to know the basic implementation and we go over how to use to solve problems using those.

Here are the problems:

problem 1: CodeForces 961E

problem 2: USACO Balanced Photo

problem 3: USACO Why Did the Cow Cross the Road III

problem 4: USACO Sleepy Cow Sort.

Hi CodeForces!

Here is the final dp stream. It is shorter than the other ones (only 1 hour) because we only go over 2 problems.

There were 4 problems for the week but we only go over the 2 hardest ones [prob 3, prob 4].

As a side note, I solved Salesman from IOI 2009 and recorded my thinking/coding/debugging process. You can see that video here. It is a dp + segtree problem.

Luqman said he isn't creating a chart of teams going to ACM-ICPC this year as he isn't competing, so I added in all the teams. Let's complete the table together!

For the teams where I don't know the cf handles of the team members, I put their names instead.

Europe

Africa and the Middle East

Asia East

Asia Pacific

Asia West

Latin America

North America

Northern Eurasia

South Pacific

Hi CodeForces!

Last week I made the dp video for my class public as a test, and it seemed really useful so I have decided to make all the videos public.

The second class was also on dp because dp is a very important concept and it shows up very often.

Here were the problems:

Problem 1

Problem 2

Problem 3

Problem 4

Here is the stream with the discussion on how to come up with the solutions. (It starts at about 25 seconds)

Hi CodeForces!

I opened registration to my competitive programming course a while ago.

The first class was today on dp and I thought I would make it public as a test to see if it is helpful to make it public afterwards. The video is here. The class starts at 30 mins.

Here were the problems:

Problem 1: Codeforces 910A

Problem 2: Codeforces 22B

Problem 3: USACO Taming the Herd

Problem 4: Codeforces 909C

Note that some of the problems had other solutions, but we discussed the dp solution to problems 1, 2, & 4.

For problem 3, I had already made a video about it and we were running out of time so I skipped it. I made a slightly different video for that problem than I usually make; it was me solving it after seeing it for the first time (basically my thinking process and later my debugging process). I've gotten feedback that this style of videos is better than my other videos, so I'll try to make more videos like this one in the future.

Hi CodeForces,

As promised, I am opening registration today for a competitive programming course.

Dates: The classes will go from January 12 to the March 16 (10 classes total). Each class is 1hr 30 mins.

Times: Saturdays 8:30am-10am, 8am — 10am with debugging (PST).

Course:

Based on interest, I am opening course 2.

Course 2: recommended level [1200-1700] most problems: [1300 — 2000] The topics are dynamic programming, depth first search, data structures, binary search, two pointers, implementation and math.

Each class there will be a set of 3-5 problems depending on the difficulty. We will discuss the solution to each problems and the thinking process to arrive at that solution. You can see the my screen, as I am writing on a whiteboard. You can ask questions while the solution is being presented.

There is also an additional debugging section. I noticed that there are people who need help debugging their codes so 30 mins before each class starts (except the first one), I can help you debug your codes from the previous week. The only downside is that the number of people who can sign up for this is limited.

If you are interested, register here! Note that on the previous post, the form was a survey for times. This is the official registration.

I will send out an email to the first few people who register for debugging help letting them know that they got in (and a separate email to those who didn't get in). Unfortunately, if there are too many people, I won't be able to help everyone so I am limiting the sign ups. The first few people who sign up for debugging get in for sure. I am also saving a few spots for those who have been working hard solving problems and can prove it (through things like submissions).

Hi CodeForces!

A couple days ago, I posted about some online classes. I got some messages with concerns, so I took the blog down while I figured out a solution. I decided that I will make the classes completely free so that they can help more people.

Dates: The classes will go from the January 5-6 weekend to the March 9-10 weekend (10 classes total). Each class is 1hr 30 mins.

Courses:

I will hold the online course with 10 classes based on the curriculum from my website.

Here are the courses I am thinking of:

Learn C++: (not from my website, includes easy algorithmic problems) recommended level: <700

Course 1: (doesn’t include the learn c++ portion from the website) most problems: [700-1300] recommended level: [700-1000]

Course 2: recommended level [1200-1700] most problems: [1300 — 2000] The topics are dynamic programming, depth first search, data structures, binary search, two pointers, implementation and math

Each class there will be a set of 3-5 problems depending on the difficulty. We will discuss the solution to each problems and the thinking process to arrive at that solution. You can see the instructors screen, so it is as if they are writing on a whiteboard. You can ask questions while the solution is being presented.

There is also an additional debugging section. I noticed that there are people who need help debugging their codes so 30 mins before each class starts (except the first one), I can help you debug your codes from the previous week. The only downside is that the number of people who can sign up for this is limited.

This is open to people from all countries.

If you are interested, fill out this survey about what times and which course you are interested in. Note that the form is not a commitment to signing up for the class, it is just to figure out how many people are interested and at what times.

I will open up registration this weekend once I figure out what time the courses will be held.

877E - Danil and a Part-time Job is an interesting problem that requires queries and updates on subtrees of a tree. It is a great problem to learn a variation of traditional segment trees.

I created a video tutorial explaining the problem. I explained how to order the nodes so that it's possible to use a segment tree and how to do toggle updates.

Here is my YouTube channel with all the video tutorials.

I created a video solution to the problem 839C. It is a DFS on trees problem.

The tutorial includes a simple code (no defines, etc) that matches the tutorial and a line-by-line explanation of the code while I write it (helpful for those stuck on the implementation of DFS).

Here is the video solution.

Here is my channel with all the video solutions.

As a side note, I am in the process of adding OI problems (such as USACO and APOI) to Course 3 of VPlanet because I realized that there isn't anywhere to find OI problems separated by topic.

Problem 698A is a typical dynamic programming problem (without optimizations or other tricks) making it a great problem to learn dynamic programming from.

I created a video tutorial for those who are struggling with dynamic programming. I realize that a lot of people already know dynamic programming but there are also many people who this tutorial could benefit.

I hope this is useful and let me know what you think!

Here is the channel with all the videos.

Last week I posted about a new website I'm starting: http://vplanetcoding.com.

I added a dynamic programming video tutorial for 855B - Marvolo Gaunt's Ring. I explained the idea behind dynamic programming and how to use dynamic programming to solve the problem.

Here is the solution. I sped up the end a little bit to keep the video short.

Unlike the DFS video I made, I didn't feel the need to explain my code because I explained it as I solved the problem. I did, however, make sure I used the same variables, loops and formulas in the code as I used while explaining the solution to the problem.

In addition, I am in the process of adding a bunch of Codeforces and USACO problems sorted by topic and difficulty to the website. I have added several problems and topics at every level. These topics include tree traversal, DP on trees, binary search, and loops. These topics have a problem but I haven't added video tutorials yet.

Here is my YouTube channel with the video tutorials.

I noticed that there aren't too many video tutorials for Codeforces problems. There are text editorials but the solutions can be hard to understand and/or implement. Because of this, I decided to create a website with video tutorials of Codeforces & USACO problems. I'm also dividing the problems by topic and difficulty.

The website is a work in progress. Here is the website: https://www.vplanetcoding.com/.

I started this website by taking one problem, 893C - Rumor, and creating a video tutorial explaining Depth-First Search(DFS) and connected components while solving the problem. At the end of the video, I wrote the C++ code without complex syntax and I explained the code, line-by-line, as I wrote it.

I think that explaining the code for the solution will help a lot of people implement the solution.

I know that many people know DFS and connected components, but there are many people who don't know DFS and connected components who this tutorial will benefit.

Here is the video: https://www.vplanetcoding.com/course-2-standard-algorithms-and-data-structures#1.

Let me know if you have feedback. Also, let me know which problems I should create video tutorials for next.