E869120's blog

By E869120, history, 5 weeks ago, In English,

Dear Codeforces community.

AtCoder Beginner Contest 106 will be held at Saturday, August 18th, 21:00 JST.

Here is some contest informations:

  • The contest is rated for participants whose rating is 1199 or lower.
  • The writers are: E869120 and square1001. We are 16-years-old twin brothers :)
  • Duration is 100 minutes.
  • There are 4 tasks in this contest.

Let's participate and enjoy for this contest, and discuss the problems after the contest.
Good luck and have fun!
---
E869120, square1001

UPDATE
The contest is over, and editorial is out!

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By E869120, history, 6 weeks ago, In English,

AtCoder Beginner Contest 105 is ongoing now, from August 11, 2018, 21:00 JST.

The writers are DEGwer, drafear, square1001 and me.
The point values are: 100 — 200 — 300 — 400.

This contest will be unrated because of problems in English statement. At first, there is no English statement in problem A. 6 minutes after starting the contest, the English statement appeared, but it was too late. Sorry for inconvenience.

After the contest, let's discuss the problem.
Thank you for your participation.

UPDATE

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By E869120, history, 3 months ago, In English,

Hello everybody!

I'm delighted to invite you to anniversary round AtCoder Beginner Contest 100, which starts at Saturday, June 16, 2018 at 21:00 JST. The time is as usual as previous round. (However, some of AtCoder contest start time is unusual...)

This is the fifth round for a Japanese twin-brothers: E869120 and square1001. I already held AtCoder Beginner Contest 064, 076, 088, 096, and this time, it's 100. Multiple-of-four again, again and again! (Where is the probability 1/1024 ???) Anyway, ultimate thanks for rng_58 for reviewed and refined my problems, chokudai for creating one of the best contest platform in the universe. It is obvious that I could not be able to host the contest without cooperation of these two people.

The round is rated for participants whose rating is strictly lower than 1200. High-skilled participants whose rating is >=1200 is also welcomed to participate as out of competition (unrated).

Scoring is 100 — 200 — 300 — 400, as usual. The contest duration is 100 minutes.

Let's participate in the contest, good luck and high rating! Also, hope to have a ultimate-great memory in the first 3-digit round!

Finally, AtCoder managed to hold the contest which the first-digit is not '0'. Since this contest is the 100th anniversary round, so some problem can be associated with anniversary. Wish AtCoder and other contest platforms will be continue till 4-digit round :)

UPDATE

1. Judge queue is clogging now.
2. Now, judge queue is fixed. Sorry for inconvenience.
3. Editorial Uploaded.

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By E869120, history, 5 months ago, In English,

Hello, Codeforces!

On May 5th, 21:00 JST, AtCoder Beginner Contest 096 will be held.

The problems were prepared by square1001 and me (E869120). Thank you for chokudai, rng_58, evima to helping us to host the round on AtCoder!

You will be given 100 minutes and there are 4 problems with following scores: 100 — 200 — 300 — 400.

This contest is rated for all participants whose rating is less than 1200, but as usual, participants whose rating is more or equal to 1200 can take part out of competition.

Good luck and have fun!

UPDATE
  • The contest will start in less than a day.
  • The contest is over! Thank you for your participation! Let's discuss about the problems.
  • Editorial is out! Link

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By E869120, 5 months ago, In English,

Square869120Contest #5 will be held on Sunday, April 15th, 20:00 JST.

About Square869120Contest

  • This contest is unofficial.
  • Square869120Contest has been held 4 times before, and this is the 5-th contest of Square869120Contest.
  • From the 3-rd contest, both Engish and Japanese statements are prepared.

Contest Information
  • Time: Sunday, April 15th, 20:00 JST
  • Duration: 240 minutes
  • Tasks: 9 (Consists of 8 algorithm tasks, 1 marathon task)
  • Writer: E869120, square1001
  • Rated: No (unrated)
  • The first problem is as easy as Codeforces Div2 A, and the last problem is as hard as Codeforces Div1-D,E problems. In addition, there are many partial scores. I think all participants can enjoy the contest.

Scoring
Task ID Max Score Partial Scores
A 200 100+100
B 300 70+140+90
C 500 90+100+310
D 600 120+160+320
E 800 40+160+190+410
F 1000 60+60+250+630
G 1200 80+320+[0 ----- 800, 1-point unit]
H 1400 210+320+870
I 1500 Marathon Task, 1-point unit

Past Contests
Square869120Contest is one of the unrated contest which have a long history. There's 4 previous contest, so you can solve them to practice this kind of contest.



Updates
1. Some of the partial scores has changed. Please check.
2. Some of the partial scores has changed. We think the point values are determined.
3. The contest is over. Thank you for your participation!
4. English editorial will be published on Thursday, 4/19/2018.
5. English editorial uploaded. Link

We are looking forward to your participation! Let's participate and enjoy!!!!!

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By E869120, history, 7 months ago, In English,

Hello Codeforces!

On February 18th, 21:00 JST, AtCoder Beginner Contest 088 will start.

Announcement Page

The problems were prepared by square1001 and me (E869120). Thanks to chokudai and rng_58 to helping us to host the round on AtCoder!

You will be given 100 minutes, and there are 4 problems with the following scores: 100 — 200 — 300 — 400.

This contest is rated for participants whose rating is lower than 1199, but as usual, participants whose rating is more or equal to 1200 can take part out of competition.

Good luck and have fun!

UPD: The contest is over. Let's discuss about the problems.

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By E869120, 9 months ago, In English,

Hello, CodeForces!

AtCoder Beginner Contest 081 (Link) and AtCoder Regular Contest 086 (Link) will be held on December 10th, 21:00 JST.

The writer is semiexp and someone.

Let's participate and enjoy. Let's discuss after the contest.

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By E869120, history, 10 months ago, In English,

AtCoder Regular Contest 085 / AtCoder Beginner Contest 078 held on Saturday, 21:00 UTC+9.

Let's discuss about the contest.
Sorry for announcing too late. (Actually, no one write about this so I wrote this blog.)

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By E869120, 11 months ago, In English,

Dear Codeforces.

This afternoon, I read the announcement of Codeforces Round #444, and I understood about the unrated incident, and the reality of the downvotes — The topic rating was -489.

Actually, I already wrote problems in 6 contests in AtCoder — 4 unofficial contests (Square869120Contest) and 2 official contest. At first, I thought that writing problems is also fun, and becoming the writer is also a honor.

But I saw about so many downvotes — even this may completely changed the writer's life. In reality, his contest has so many mistakes in a single contest, but I knew that the writer could be the worst contributor out of 20000 people.

So, today, I felt fear to writing problems and being the writer.

Human always have a probability to make some mistakes. Fortunately, there are no serious bug in my contests, and all official contests which I already wrote was rated. But any time any people can make mistake. So all of the writer always have probability to fail and get -500 downvotes, and finally they may die in competitive programming.

What is your opinion? I want to listen others' opinion.

E869120

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By E869120, 11 months ago, In English,

Hello Codeforces!

I'm quite excited to invite you to AtCoder Beginner Contest 076 which will held on Saturday, October 28th 21:00 JST and lasts for 100 minutes.

The round problems are created by me (E869120) and square1001.

This contest will have four problems and it is rated for contestants whose rating is below 1200. The same as before, contestants whose rating is more than or equal to 1200 can take part out of competition.

The scoring is: 100 — 200 — 300 — 400.

I hope you can have fun during the contest. Good luck and have fun, wish you high rating!

See you tomorrow!


UPD 1: The editorial is here. Thank you for participating!

UPD 2: Congratulations for the winner!

Rank Username A B C D WA/TLE Total
1 mamekin 00'54" 02'21" 07'30" 16'04" 0 16'04"
2 uwi 00'58" 03'00" 10'05" 19'40" 0 19'40"
3 dreamoon 01'29" 02'50" 07'28" 17'41" 1 22'41"
4 sugim48 00'37" 01'47" 05'11" 18'06" 1 23'06"
5 kobae964 00'59" 02'05" 06'38" 18'09" 1 23'09"
6 Warden 02'18" 04'49" 10'39" 23'31" 0 23'31"
7 TangentDay 00'37" 02'13" 06'13" 24'32" 0 24'32"
8 cyand1317 24'51" 24'51" 24'50" 24'50" 0 24'51"
9 femto16 01'26" 03'14" 08'45" 25'10" 0 25'10"
10 iehn 01'16" 02'25" 07'38" 27'26" 0 27'26"

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By E869120, 12 months ago, In English,

Dear Codeforces.
Today, I wrote about a mystery of Codeforces contest start time.

1 — Background

There are many codeforces users from many countries. In spite of this, recently I feel Codeforces contest time is too biased and it's difficult to participate or enjoy for some country. For example, Codeforces Round #434 started at 13:05 UTC, Round #433 started at 12:55 UTC and Round #432 started at 14:35 UTC. It have only around 3 hours difference between "the earliest of the 3" and "the latest of the 3".
This is also an answer of "why I wrote this blog".

2 — Codeforces Time Distribution Graph

First, to investigate actual codeforces time, I searched recent 100 contests time and made a distribution graph.



3 — Problem

Recently, I found that Codeforces Contest times are biased and they're difficult to participate for some people:

  • Around 9000 out of 30000 (30%) active users are difficult to participate because they're too late time. (China, Japan, South Korea, etc.)
  • Around 3000 out of 30000 (10%) active users are difficult to participate because they're too early time. (United States, Brazil, Canada, etc.)

For example, the standard time in Beijing is UTC+8, so around 65% of contest starts at 22:00 or later, and ends at 24:00 or later. A large amount of participants are students, and I think 24:00-03:00 is too late time for student. If they saw systemtest and rating change, the sleeping time could be one hour later.

The other example is in New York. The standard time is UTC-5, so around 95% of contest starts at 12:00 or earlier, and ends at 14:00 or earlier. I think it is too early time, and the time overlaps for most people because of school or working. They can wake up more earlier, but I think morning is busy time for the most, so it's difficult to make time to participate a single contest in morning.

4 — My Suggestion

I think it is better that codeforces do this:

  • ~20 or ~30 percent of contest should move in range 06:00-12:00 UTC. It's better time for Chinese, Korean, Japanese, etc. (It is also possible time for Indians, etc.)
  • ~15 or ~20 percent of contest should move in range 21:00-03:00 UTC. It's better time for American, Canadian, Brazilian, etc.

This system in my suggestion is like TopCoder. (Contest time is not too biased)

I think this made some good effects and bad effects as follows:
  • It became just a little bit more difficult to participate Codeforces contest for Indian, Russian, etc.
  • It became more easier greatly to participate Codeforces contest for Chinese, American, etc.
  • On the whole, I think active users would increase because it seems that there are many people who didn't cannot participate because of time.

This is just my suggestion, but I think this is also a strategy to increase users and to make codeforces more active.

Thank you for listening.

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By E869120, history, 12 months ago, In English,

AtCoder Regular Contest 083 (Link) and AtCoder Beginner Contest 074 (Link) has started, and ends at 13:40 UTC.

This time, no one wrote the announcement page and I felt that many people want to discuss about it, so I wrote this page for discussing and talking.

After the contest, let's discuss about the problem.

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By E869120, 13 months ago, In English,

TopCoder SRM 720 will starts in ~8 hours.

Time: 8/24 07:00 EDT, 8/24 11:00 UTC, 8/24 14:00 MSK
Duration: 75 minutes coding phase + 5 minutes intermission + 15 minutes challenge phase

You can check the future contests from calendar.

Let's discuss after the contest.

Final Results

1. Division 1

RankCompetitorTotal Point
1yosupo1126.37
2Petr743.11
3tourist733.44
4sky58722.65
5nika710.03

2. Division 2
RankCompetitorTotal Point
1r3gz3n782.27
2mariandz781.66
3Trias757.82
4geek_geek750.79
5sachintcthope739.68

Congraturations to winners!

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By E869120, history, 14 months ago, In English,

Today, Codeforces Judge is stooped, and now judge queue length is very long.



  • Judge is stopped more than 13 hours. This is as long as CF normal round x6.
  • 5000+ submissions are In Queue.

This is a mystery of CF Judge Queue. Why these things can be occur?

UPD 1: Now Codeforces Judge is moving again.
UPD 2: Now Judge Queue is moving as usual, and solutions are judged within 5 minutes after submissions. Thank you for Codeforces!

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By E869120, 14 months ago, In English,

Dear Codeforces Community.

Today I want to share some ways to practice competitive programming and getting rating. I think this is helpful for those who is practicing competitive programming hardly but rating is sluggish. (By the way, on July 17th, I have a project of competitive programming said CombNaf in Japan. I did a lecture about this. Great thanks to the CombNaf's organizer is Nafmo2.)

I will write this by 4 steps: rating 1000 --> 1250, 1250 --> 1500, 1500 --> 1750, 1750 --> 2000, in Codeforces Rating System.

Before writing about each step, I wrote it as premise: You don't have to do this way. This is just a way to practice. Ways to practice is different among people, so I think this may not the best, but I hope this is useful.


Step 0: Some types of contest (A knowledge)

In order to explain step 1-5, I wrote about the types of programming contest.

Codeforces

  • This judge. There are Div.1 problems and Div.2 problems. The number of contest is mainly 5-6.
  • The problems of Div.2 said Div2 A, Div2 B, Div2 C, Div2 D, Div2 E,... in order.
  • The problems of Div.1 said Div1 A, Div1 B, Div1 C, Div1 D, Div1 E,... in order.
  • Problems are sorted by difficulty in each contest.

AtCoder
  • There are ABC (AtCoder Beginner Contest) / ARC (AtCoder Regular Contest) / AGC (AtCoder Grand Contest) in AtCoder, but in this blog I will explain about ABC / ARC.
  • There are 4 problems in ABC and ARC.
  • Each problem in ABC is said ABC-A, ABC-B, ABC-C, ABC-D, and each problem in ARC said ARC-C, ARC-D, ARC-E, ARC-F.
  • Problems are sorted by difficulty.
  • In each contest, ABC-C and ARC-C is the same problem, and ABC-D and ARC-D is the same problem.

TopCoder
  • There are Div.1 and Div.2, and there are contest for each division.
  • In Division 2, there are three problems, which is said that Div2 Easy, Div2 Medium, Div2 Hard.
  • In Division 1, there are three problems too, which is said that Div1 Easy, Div1 Medium, Div1 Hard.
  • Easy is the easiest question of three, and hard is the hardest question in these three as naming.


Step 1: Rating 1000 --> 1250

In order to gain rating from 1000 to 1250, you should solve at least one problem in Div.2 contest in Codeforces. In AtCoder, 300 points problem is the level of rating 1100-1250. So I suggest these two ways:

  • Solve Div2 A 50 problems. When you solved 50 problems, you might be able to solve >80% of Div2 A.
  • Solve ABC-C in AtCoder. There are many educational problems in AtCoder Beginner Contest.

In order to solve problems, you should make a Bingo like example.
In addition, most of these problem is easy, especially concept. So you should see editorials if you can't reach idea 10 minutes.

Step 2: Rating 1250 --> 1500

In order to gain rating from 1250 to 1500, you have to solve at least 2 problems faster in Div.2 contest. In addition, the level is as same as TopCoder Div2 Med and AtCoder ABC-D. (ABC-D is little high level for 1250) In addition, there is many educational problems in AtCoder, there is some point to do fast-solving practice in TopCoder, and Codeforces is the target judge. So I suggest these three ways:

  • Solve Div2 B 50 Problems. (Most of problems are good quality)
  • Solve Div2 Med 50 Problems. (The quality of problem is good, but Java Applet is inconvenience...)
  • Solve ABC-D / ARC-D in AtCoder. (A little high level for 1250)

In addition, I think that you should mind fast-solving in latter problems. (After solved 15-30 problems)
In order to mind fast-solving, you should use timer. You should count from "opening problem statement" to "getting AC". If you can, I think that you should make a spreadsheet of problem, solved and time.


Step 3: Rating 1500 --> 1750

In order to gain rating from 1500 to 1750, you have to solve at least 3 problems faster in Div.2 contest. There are a lot of concept problems in Div1 A = Div2 C, and in Div2 only contest you have to solve as fast as possible. I made a table of judge and points to see what to solve easier.

Judge Concept Imprementation Fast solving Level
Codeforces Div2 C 50% o 50% 1500-1800
TopCoder Div1 Easy o x o 1500-2000
AtCoder ABC/ARC-D 50% o 50% 1400-1600

I suggest these two ways to improve rating as far as see the table:

  • Solve Div1 Easy and Codeforces Div2C as the same period. I think if you solve <50 problems for each type, your rating will increase strongly, but I suggest you should solve until satisfied yourself.
  • First solve ABC/ARC-D in AtCoder until solve 80% of ARC-D. Second solve Div1 Easy in TopCoder for concept-practice or fast-solving practice.

My rating increased sharply when I started TopCoder Div1Easy, and solved ~50 Div1Easy problems. This is why I suggest TopCoder Div1 Easy for concept-practice.
In addition, you should use timer for practicing fast-solving. You can use competitiveprogramming.info to solve TopCoder Div1Easy, and you can make spreadsheet like following picture to solve TopCoder. (This is example of Div1Med that I am using.)



Step 4: Rating 1750 --> 2000

This is the last step that I can write. In order to gain rating 1750 to 2000, first you must go up to Div1, and you have to compete a little better in Div1. You have two steps, so I divided into two range.

1. Rating 1750 --> 1900
You should solve Div2C faster and stably. So I suggest that practice these two:

  • Overcome your weakness (For example, DP problems, Graph Theory, Imprementation, etc.)
  • Make your library (For example, RMQ, BIT, Segment-Tree, etc.)
I think making library is good because you can shorten the time that writing RMQ class, BIT class, etc.
And to overcoming your weakness, I suggest that analyze your time in contest and practice, scoring and make a spreadsheet as follows:



2. Rating 1900 --> 2000
This step's range is only 100, but I think this is difficult as far as see A mystery of CF rating distribution. There are many people in [1900, 2000), but there aren't many people in 2000+. In Div1, there are many concept-main problems. So I think these two are useful for practice:

  • Codeforces Div1 B. In the story, the goal is becoming 2000+ in Codeforces. So practicing in Codeforces is the best too to get rating in CF.
  • AtCoder ARC-E. ARC-E is 600-900pts in AtCoder, and this is level of rating 1900-2200. In addition, these problem is very like to Codeforces.

These problems are so difficult, so I suggest that you should give up and see the editorials if you can't get any idea though you try over 80-150 minutes. In addition, ARC-E is difficult for 1900, so I think you don't have to mind fast-solving.


Step 5: Extra corner

In extra corner, I suggest two ways to compete well in Codeforces. This is also out of the problem-practice, but I think this is effective. (I did this and I feel this is effective.)

  • Do Virtual Contest / Virtual Participation in Codeforces. This is a way of get use to contests.
  • Take a rest for 10 minutes before real contests. This is a way to not get panic in the contest. It is also important in the contest on the mental side.


Conclusion

I suggest that five steps to practicing competitive programming. Ways to practice is different from a person to a person, so I don't think you must do this way. But this is one of the effective way I guess. (I think this is not the best because the way to fit is different among people.) I hope it will be useful even a little. (Also, sorry for my poor English.)

Please comment if you have suggestions and questions of this entry, and my way to practice.

Thank you for reading!

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By E869120, 15 months ago, In English,

Recently, I realized a funny incident.
The incident was happened for two days.

Day 1 (June 28, 2017)

In this day afternoon, I realized 3 out of 7 my blogs was downvoted, though there is no blog in recent actions. It is sad, but some of this is so funny: The number of downvotes in all 3 blog was "-10".

1: A mystery of CF rating distribution (Link)


2: A mystery of CF contribution distribution (Link)


3: A mystery of correlation between rating and contribution (Link)


All of these are -10 votes differ! Why!?!?!?

The second story in this day is at night, which is a bit happy.
There is much upvote in a comment, though there is no recent action! (In addition, the differ is "+10".)

(Also, I felt suspicious about this because I think tomorrow maybe happen an incident about "10".)



Day 2 (June 29, 2017)

As I expected, an incident happened in day 2 afternoon.
There is some downvotes in other blog, which is 2 months ago and of course there isn't the blog in recent actions.

4: Square869120Contest #4 Editorial (A-D) (Link)

OH! SOMETHING HAPPENS! I expected -10 votes as previous three so the blog post rating is going to +19 -> +9, but this is +10. Interesting. (Note: All of -9 downvotes happened in 5 hours.)

=============== AFTER 4 HOURS ===============

Finally, the blog's rating decrase by 1. As I expected, the number of downvote was "10".




Summary

The incident is so mysterious and funny. A mystery about the elected number: "10".

In one sentence conclusion: "+10, -10, everywhere!".

Thank you for reading.

UPD: Thank you for gongy to react my funny story and write a blog. (Link) Thank you very much.

Update (Day 3)

Finally, the blog rating was temporary +16, but this was -50 at the end of Day 3.
In addition, the blog's rating change is really interesting as follows:

Time elpased Upvotes / Downvotes
0 min 0
15 mins +7
18 mins -3
60 mins +16
8 hours 0
9 hours +2
14 hours -19
18 hours -7
24 hours -30
36 hours -50

I hoped that the incident won't got worse, but it was worse very much. The number of downvotes in the incident was:
Funny story Myst. Vol 4 (deleted) Myst. Vol 3 (deleted) Myst. Vol 2 Myst. Vol 1 s8pc editorial Total
-60 (0 -> -60) -47 (+25 -> -22) -25 (+50 -> +25) -51 (+114 -> +63) -25 (+154 -> +129) -10 (+19 -> +9) -218

The next episode (July 22th)

The incident seems to be end, but this happened again. At 7/22 20:00 JST, suddenly, most of my blog downvoted! I thought why this reason is, so I decided to update this blog.

Tutorial Funny story Myst. Vol 2 Myst. Vol 1 Total
-20 (+180 -> +160) -15 (-60 -> -75) -20 (+63 -> +43) -20 (+129 -> +109) -75 (Total -293)

Funny Story with "20"!?
I think this incident is so serious, and I never heard a bigger downvote incident than it in CodeForces. I really want who made 10 fake account and downvoted me. Making fake account is also an immoral thing, so please comment this blog if you make 10 fake account.

By the way, thank you for reading!

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By E869120, 15 months ago, In English,

Recently, I'm interested in CF rating and contribution distribution, and I searched this.
I already searched about mysteries of rating distribution (Blog), so I wrote here about contribution distribution. (Thank you for comments)

Method of Searching

  • Here is the leaderboard of top contributor.
  • I searched the rank of contribution -51, -46, -41, -36,..., -1, 4, 9, 14,..., 164, 169, 174, 179. I searched the number of contributors too.
  • Then, I make the distribution graph.
  • Each range of the graph is [-inf, -50), [-50, -45), [-45, -40),... , [175, 180), [180, inf]. Each value of x-axis of the graph is the minimum value in range. For example, if the value of axis is 15, this means the range is [15, 20).
  • The minumum value of range [-inf, -50) is inf, but in distribution graph this value is -1000.

However, there was some surprising thing about it.
Look at following graph:

Note: The distribution graph has Logarithmic scale. Every time the scale increases by 1, the value doubles (2 times).

The surprising things are following:
  • Obviously, the number of people who have contribution [100, 105), [105, 110), [110, 115) is especially larger than [90, 95), [95, 100).
  • The number of people decreases greatly from contribution [10, 15) to [15, 20). The difference is approximately 2.5 times.
  • The number of people who have contribution [-50, -45) , [-45, -40), and [-40, -35) are nearly the same.


Why these things can be occur?

UPD1: rng_58's contribution became 180, so the graph was extended. Wonderful.
UPD2: The distribution graph is extended because some user's contribution was changed.

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By E869120, 15 months ago, In English,

Recently, I'm interested in the rating distribution and I searched this.

Method of searching

  • Searching the place of people (in active users) who has rating 999, 1049, 1099, 1149, 1199,... , 3049, 3099.
  • Let pn be the place of people who has rating n.
  • The number of people in range [1000, 1050) is p999p1049, the number of people in range [1050, 1100) is p1049p1099, ...

However, there was some surprising thing about it.
Look at following graph:

  • The number of people who have rating [1900, 1950) is especially larger than [1800, 1850), [1850, 1900). ( twice larger approximate )
  • The initial rating is 1500, but rating [1350, 1400) is about twice larger than rating [1450, 1500).
  • The average rating is about 1450. This is lower than the initial rating: 1500.

Why these things can be occur?

UPD: The graph was extended.
UPD2: The average rating was calculated.
UPD3: The distribution and searchings about codeforces contribution is Here.

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By E869120, 17 months ago, In English,

Hello, everyone.
I'm sorry for writing editorial too late.
I hope that you can use the commentary to solve the problem of s8pc or review the contest.
Please write comments where you do not understand, impressions, and discussions.

This is the square869120contest #4 (in AtCoder) editorial.
Writers were square1001 and E869120.

Since I think that there are also many people who have not yet seen the problem and forgotten it, I will attach a link to the problem statement.

A — Atcoder Handles

In this problem, the possible index of the handle name of Mr.X will be a "Range". (Example of "Range": {4,5,6,7}, {3,4,5,6,7,8,9,10})
Proof:

  • The handle name of Mr.X doesn't include '?' — This means there is only one possibility of his name.
  • If one of the others name (in the list) changes, the index of Mr.X will change no more than one.
  • So, every index between "Minimum" to "Maximum" is possible.

For example:
 abcde -> zzzzz(changed)
bbbbb
ccccc
ddddd
Mr.X: cprpr
In this example, Mr.X's index changed from 4 to 3.

Adittionally, The maximum and minimum value of Mr.X's index can find for this way:
  • Minimum: When all characters of '?' in the list are 'z'.
  • Maximum: When all characters of '?' in the list are 'a'.
Since possible index of the handle name of Mr.X is "Range", you can solve this problem with only minimum-value and maximum-value.


B — Buildings are Colorful

Let's consider the case of N = K.
You can solve this problem with greedy method if N = K is true. (a[i] is the height of building i)

 long long ret = 0, Y = 0;
for (int i = 0; i < n; i++) {
if (Y >= a[i]) { ret += ((Y + 1) — a[i]); Y++; } // When previous building (After change) is lower than this
else { Y = a[i]; } // Else
}
//The answer is ret.
So, you can get 120 points in this way.

Let's think about "What buildings should be visible". (The number of should-visible buildings must be greater than K)
There are N buildings along the line, so you have to think only 2N ways.

You can use greedy method to solve the following problem:
  • You are given the index of buildings that should be visible.
  • Find the minimum cost.
The greedy method is here:
 Let L be the maximum value of building's (that in front of this, After changed) height:
If the building don't have to be visible, this building's height will not be increased.
If the building have to be visible, this building's height will be max(This building's height,  L)
You can solve this problem (Max point) with brute-force method and greedy method.
The complexityis O(2N).


C — Calendar 2

You can solve this problem in O(N) in BFS or DFS algorithm, but you can only get 100 points in this way.

Let's consider to split the calendar every M rows. (Because this calendar have Periodicity per M squares, if m in the problem statement is divided by 7, M will be m / 7)
"The calendar between i * M + 1-th row to (i + 1) * M-th row" and "The calendar between (i + 1) * M + 1-th row to (i + 2) * M-th row" are the same.
So, you can solve this problem following way:

 Let "number of connected white parts between small calendar between 1st row to M-th row" be A
Let "number of connected white parts between small calendar between 1st row to M * 2-th row" be B
The answer is (B - A) * (N / M) + (2 * A - B).
The complexity is O(M) + O(M) = O(M).


D — Driving on a Tree

You can use the Tree-DP method in this problem.
This is a typical problem which use "Every-Direction Tree DP Algorithm" (It says "全方位木DP" in Japanese and translated directly, actually I don't know English word of the algorithm). The similar problem is 790B.
Let's consider the rooted tree which the root is node 1.

Let dp[i] be the expected value of number of moves which only go down (= to children of the node) and starts on node i
=== This value can find with Tree-DP algorithm in O(N).
Let dp2[i] be the expected value which starts on node i and go to the parent of the node first.
=== This value can find with Tree-DP algorithm (From the root of the tree to leaves) in O(N).

  • Let the parent of node i be j.
  • Let the degree of node i be p.
The answer of node i is average of {dp[i], dp[i], ..., dp[i], dp2[i]}. dp[i] comes p - 1 times. The complexity is O(N) + O(N) = O(N).


Thank you for looking this page.
I hope that you can use the commentary to solve the problem of s8pc or review the contest.

Please wait for a while for editorial between problem E to H.

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By E869120, 17 months ago, In English,

In Apr/09/2017, there was a contest in Atcoder.
The contest name is "Square869120Contest #4". Click here

How was it?
Please write and discuss here about impressions and "qustions about solutions".

Thank you for everyone.
It is also advisable to solve those who are not participating or unsolved problems as past problems.

=========================================================================================================
past Square869120Contest
Square869120Contest #4:Click here
Square869120Contest #3:Click here
Square869120Contest #2:Click here
Square869120Contest #1:Click here

Update: If you can, please answer the questionnaire for the improvement to s8pc-5.

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By E869120, history, 18 months ago, In English,

Square869120Contest #4 will be held on Sunday (4/ 9) 11:00 UTC. (20:00 JST / 14:00 MSK)

About Square869120Contest
・This contest is unofficial.
・Square869120Contest has been held 3 times before, and this is the 4th contest of Square869120Contest.

About Contest
・The contest are prepared by 9-th grade students, E869120 and square1001.
・Let's participate and enjoy!

Information
・Time: April 9th (Sunday), 20:00 JST (11:00 UTC)
・Duration: 200 minutes (3 hours 20 minutes)
・Number of Tasks: 8 Problems
・Writer: E869120 and square1001
・Rated: No [unrated]
・Language: Japanese / English
・This contest has many partial points, so if you can't get perfect score, you can get many partial points!

Past Square869120Contests
Square869120Contest #3 (English / Japanese)
Square869120Contest #2 (Japanese)
Square869120Contest #1 (Japanese)

Update: The penalty of WA / TLE (or something unsuccessful submission except Compile Error), changed from 10 min to 0 min, so you don't have to worry about WA penalty!

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