2025 CSP-J/S Travelogue

By longyitongxue

Posted: 2025-07-16 13:45

Category: Life / Travelogue

Early July 2025

I received the notification to register for CSP-J/S and logged into the NOI website. This was my first time participating in "eating chips" [1], and I was so excited. Speaking of eating chips, I started to envy gjt [2]. At the same time, I had been studying and doing problems during this period (my count of green-rated problems passed also started increasing from here, although not by much).

July 18, 2025 — August 3, 2025

17 consecutive days of training camp (details in this training camp travelogue), I gained a lot and also shed a lot of sweat. During this time I even had to change my password because someone saw it.

Preliminary

Before the Preliminary

While doing the Chu-Su summer homework, I also worked on the preliminary round section, competed on Luogu, did problems on Luogu YouTi, and learned about segment trees. The number of green and blue problems I passed kept soaring.

But later I didn't get good results in two consecutive Luogu SCP mock contests, and I didn't reach the cutoff line in two consecutive CSP-J 2019 attempts. After thinking about it,

[Screenshot: Luogu post by longyitongxue on 2025-08-14]

"Hey, I just thought about it. I shouldn't count on hooks anymore. I shouldn't count on making it to the finals. First, prepare for the preliminary round. Stay calm, do more mock tests. You might not get good results at first, but you'll definitely gain something."

I analyzed and found that my weak points were in reading programs and completing programs.

[Screenshot: CSP-J 2019 score — 61.5 points]

(CSP-J 2019 performance... I won't say much about it) (Please give a little follow for my sake as a noob)

So for these two days I put down segment trees (what about S group, lol) and went to study reading programs and completing programs.

Let me show some of my records.

[Screenshot: Luogu submission records showing scores from 61.5 to 88]

(The first one, 61.5, is from 2019, extremely hard; the last one, 88, is from 2024, the 2024 problems were so easy I have no words)

[Screenshot: handwritten notes on scratch paper]

(Showing you my scratch paper, it's very messy don't learn from me)

Then I got tired of doing J group problems so I tried S group.

[Screenshot: SCP-S score — 55 points]

Also easily passed the cutoff line.

You have to do problems on YouTi, and also the Yibentong preliminary section.

Then came a shocking moment:

[Screenshot: Email about error in Yibentong preliminary book]

Subject: Error Found in "Information Olympiad Yibentong Preliminary Real Question Analysis"

In the book, the second sub-question of the [NOIP 2011] program reading true/false section clearly has the correct answer as False (x), and the analysis also says "after removing, tel[i]-'A' may cause array out of bounds", but the answer given in the book is True (checkmark). Hope this error will be corrected in the next edition.

Response from Dong Yongjian on 2025-08-17: "This error will be corrected in the next edition. Thank you for your feedback."

One Week Before the Preliminary

gjt AFO'd a bit unexpectedly.

Junior high school life has arrived, so much homework!!!

Under the pressure [4] of whk [3], I still made time to do some problems. (But after the preliminary there's a monthly exam next week, pressure is huge)

[Screenshot: Luogu comment by libu2333 on 2025-09-12]

"AAAAAAAAAAAAAAAAAAAAA!!!! @Natural_Selection: Thunderbolt from the blue!! @Natural_Selection: Had a mental breakdown in an instant!! @longyitongxue: 7 days left until CSP-J/S 2025 Round 1"

The preliminary is coming soon, seems everyone is stressed. It's fine, stay calm, let things take their course. If worst comes to worst, try again next year (but having to wait another year aaaaaa).

Thanks to the blessing of chenshiyujingxuan:

[Screenshot: Luogu comment by chenshiyujingxuan on 2025-09-14]

"Not made by me, but I demand putting you and kaka together (@an: Having my good review, and sticking with kaka is even better. @chenshiyujingxuan: ..."

You have to do problems, doing the first and last ones is always good.

[Screenshot: CSP-J new format preliminary mock test questions]

[Screenshot: Yibentong Preliminary section table of contents]

(J group set 15, about the recursive version of 01 knapsack I actually spent half an hour to figure it out, the last completing program I got all correct using the old method of leaving space above and below)

Let's do some S group problems too, pick the middle ones.

[Screenshot: SCP-S results]

3 Days Left Until CSP-J/S 2025 Round 1

[Screenshot: countdown image]

(Due to some reason, Luogu's countdown is one day earlier, same for the finals, they add it all to October 31.)

Great, I have to write a reflection essay, great, ugh, this is the 6th time since I entered 7th grade that I've written a reading/watching reflection... (planting a foreshadowing here secretly)

After thinking about the essay, went to study basic knowledge.

2 Days Left Until CSP-J/S 2025 Round 1

[Screenshot: countdown image]

Great, wrote an 800-word essay today, great. The foreshadowing from before, let me reveal it: tomorrow afternoon periods 8,9 we have to watch the Qian Xuesen biography movie, and tomorrow our OIers happen to have OI in periods 8,9, and we also have to write a reflection essay, and it's due next week. Asked the teacher, can make up watching on the weekend, but who has time??? Saturday is eating chips + OI + ABC [5], Sunday is OI study, I'm dead aaaaaa 555! Targeting us OIers! Hope DP [6] can help me out...

After finishing whk homework, have to go back to eating chips. (Really don't want to do math homework, a bunch of random calculations, today repeat this problem, tomorrow repeat that one, I've memorized them all backwards, except for carelessness there's basically nothing to talk about)

@[MingDynasty] gave inspiration not only to the OP but also to me under a post, read this summary. The preliminary is coming soon, although I've prepared a lot (there are many of my efforts I didn't screenshot) but let me still review a bit more since getting into the finals means I can grab silver [7], this is the result I've been working hard for 3 months to achieve!

1 Day Left Until CSP-J/S 2025 Round 1

[Screenshot: countdown image]

Today is also 9/18, so let's remember history:

[Screenshot: doodle drawn during art class]

Drew this during boring art class, but still wish everyone RP++ ~

[Screenshot: more doodles]

Ah, all the classmates went to watch the movie, only us few OIers got locked in the computer lab to study OI 555! We still have to make up watching and make up essays!!! (Damn, fpnzlll [7] went to watch the movie behind our backs instead of studying OI seriously)

Since I'm already at the computer lab, might as well do some preliminary practice.

0 Days Left Until CSP-J/S 2025 Round 1

[Screenshot: countdown image]

[Screenshot: homework screenshot]

So much homework, targeting OIers right? Two essays!!

The Preliminary Arrives

Slept well, welcoming the incoming J group.

8:50 Arrived at longitude 113.58777023555649, latitude 24.811806336683443.

9:00 Enter the venue

9:30 J starts

11:30 J ends

This year's problems are so hard, I didn't even write down the answers to estimate my score (didn't dare, afraid of being disqualified), from what I know already lost 8 points! Dead, dead, let's see the afternoon S group, I need hooks!!!!!!!!

At noon in the computer lab chatting with classmates and checking answers on cz's post.

Afternoon continuing with S.

14:00 Enter the venue

14:30 S starts

You have no egg! caught my attention.

16:30 S ends

Afternoon S I finally learned to write down answers......

After the Preliminary

2025/9/25 Results came out.

[Screenshot: score screenshots]

Not bad.

[Screenshot: more score screenshots]

The cutoff lines came out, everyone made it to the finals!! (Last year GD preliminary J cutoff was 83.5, this year 59.5, I'm not seeing things right?)

So much homework during the break aaaaa 555! But silver [7] was secured.

[Screenshot: achievement screenshot]

Now the goal: steady 4 silver [7], rush 6 gold [8].

Finals

Before the Finals

Oh no, mock contests always go badly, never get a three-digit score, difficulty is about CSP-S level, even posted for help from big brothers, seems gold [8] is still difficult.

| Day 1 | 0 | 12 | — | 0 | | Day 2 | 36 | 13 | — | 0 | | Day 3 | 17 | — | — | — | | Day 4 | 18 | — | — | — | | Day 5 | 20 | 10 | — | — |

Table 1: Mock contest scores

Then during October I got sick twice, annoying

[Screenshot: screenshots]

[This content was forcibly deleted due to XXX reasons]

The original content contained the word "watermelon" [8], which hereafter refers to @[ouxiyao].

[Data deleted] [9]

Two Weeks Before the Finals

Doing Luogu SCP.

2025-10-18 was the S group mock contest, happened to have a fever that day, then from 12:00 to 17:30 was at the hospital, already feeling unwell, came home and frantically did problem corrections, fortunately T1 was easy...

| 100 | — | — | — | | SCP-S score |

| Puj/Tihao- | Tihao+/Shengxuan- | Tihao+/Shengxuan- | NOI/NOI+/CTSC | | SCP-S difficulty |

2025-10-19 J group mock contest, state was okay that day. First saw problem A was fine, spent about 20 minutes and got 80% confidence on it, then problem B stumped me, spent over two hours and still couldn't figure it out, then quickly looked at C, personally felt C > B, then got 70% confidence on it. During the contest thoughts: hope I don't bowl (get all wrong). Then got 100-200 points, preferably B can be guessed...

After the contest, indeed within the estimated range, also at the lowest end of the estimate, as expected...

| 100 | 0 | 0 | — | | SCP-J score |

| Puj- | Puj/Tihao- | Puj+/Tihao | Tihao+/Shengxuan- | | SCP-J difficulty |

For J group I also recorded the whole process.

[Link: BV1T6swzcEAV]

One Week Before the Finals

[Screenshot: school sports day photo]

Last week the school had a sports meet, when not competing found it boring, studied and reviewed algorithms with watermelon

[Screenshot: photo with watermelon] (watermelon is on the left, me on the right)

That night, the coach said there weren't enough double beds for the finals, need to draw lots and those who draw the short straw sleep on small beds, turns out segment tree [10] drew the short straw, and I was roomed with segment tree, great, can enjoy the 1.8 meter big bed!

3 Days Left Until CSP-J/S 2025 Round 2

[Screenshot: countdown image]

Originally had OI class tonight, just wanted to complain about yty's [11] recent monthly contest problems, someone with O(n^2) passes a million like that just brute forces it [12], I used a segment tree to maintain max and other brute force with an extra log factor and all got TLE'd, but because of homework decided to watch the replay...

2 Days Left Until CSP-J/S 2025 Round 2

[Screenshot: countdown image]

Had four math classes, laughed out of anger.

Another night with so much homework can't even log into Luogu...

1 Day Left Until CSP-J/S 2025 Round 2

[Screenshot: countdown image]

Wanted to do a big simulation to wish me RP++, got WA instead... sugar died...

0 Days Left Until CSP-J/S 2025 Round 2

[countdown image]

As a good student, I decided to bring no electronic devices to the finals, and didn't log into Luogu all day.

Afternoon PE exam needs to be retaken, then at 13:00 took a bus directly from school to Shenzhen.

[Screenshot: photo with segment tree] (left to right: segment tree, me)

Around 17:45 thought we arrived at the hotel, turned out we went to the wrong one...

18:00 Arrived at hotel, ate dinner.

21:00 Originally supposed to sleep at 22:30 but segment tree and I decided to sleep early.

Then, segment tree set the AC to 20 degrees Celsius at night, he woke up cold in the middle of the night but didn't turn up the AC, making me sleep until broad daylight, sneezing as soon as I got up in the morning...

The Finals Arrive

06:10 Segment tree's alarm went off, followed by about 5 minutes of sneezing...

06:30 Ate breakfast. (Because our coach said he would call us and we had to say "weh" or they'd kick us out of informatics, segment tree and I were scared, so we waited for the call until 06:30 then said "weh")

07:00 Depart.

08:00 Around went to take a photo under the CSP poster.

[Screenshot: photo at CSP poster]

Suddenly remembered I hadn't taken off my watch, borrowed fpnzlll's bag to temporarily store it.

08:15 Enter exam room

Can't bring a thermos, gonna die of thirst 555

08:27 Start!

First T1 and T2 were easy, just watered them

Then T3, felt so complicated, since I didn't know the inverse operation of XOR, directly used a segment tree (maybe because I slept with segment tree all night, got infected by him, whenever I see this type of problem I think of segment tree) to maintain XOR sum, then forgot how to write the range update part, recalled it on the spot but couldn't pass the extra test cases...

T4 just DFS brute force...

11:57 End, go back to hotel for dinner.

Estimated score 200+.

13:45 Depart for S group.

14:15 Enter exam room.

14:27 Start

T1 at first thought greedy, then found greedy doesn't handle postfix dependency, immediately thought of DP, then immediately didn't know how to do it, wrote a DFS to get 20 points.

T2 skipped.

T3 is an AC automaton, no problem, then KMP (the STL built-in kind) brute forced it...

T4 found special property A is fun, so wrote a combination number solution.

Went back to T2, wow, special property A is too fun, just output 0 and went.

18:27 End. Went to the bathroom, then got on the return bus.

Wow, sugar died, T2 originally guaranteed that there exists one aj,i != 0, and one cj = 0, killed me, I read it as "for all", damn!

T4 asks for combination numbers, don't know any other methods, so brute forced a factorial method, but immediately obvious that with modulo need modular inverse, since I really forgot how to write it, sigh, estimated score went from 102 to 50...

In summary: [screenshot], but not completely, because of J's later problems... never mind, this time steady 4 silver [7], rush 6 gold [8] plan seems like can only secure 4 silver [7].

Chat records from the next day:

[Screenshot: chat logs]

23:33 Arrived at school...

-1 Days Left Until CSP-J/S 2025 Round 2

[Screenshot: countdown image]

Then went to test on unofficial data

After the Finals

2025/11/5 Scores came out

[Screenshot: score screenshot]

S group didn't even reach the lowest estimated line...

So unskilled 555 compared to our school people I'm so noob... worship luudan [13] big bro AK CSP-J, Au CSP-S. Alright, this year wanted to finish in one go gold [8], but accidentally ran out of breath, fine fine fine.

Award Collection

Face reality...

[Screenshot: award screenshot]

555, a bunch of 5 silver [7] in my class, this can only mean I'm so noob...

2025-11-24 Went to whk's IT class, took the opportunity to log into Luogu, found that award collection was available.

[Screenshot: award collection screenshot]

But after getting silver [7], I found that when I didn't have silver [7], in the beef feed there were a bunch of silver [7] big bros, now I have silver [7], but the people I used to follow in the beef feed all turned into gold [8]...

Perhaps (no perhaps), there are reasons for this:

1. I'm too noob, forgot range update on the spot...
2. I was blind, didn't see the special property: in J2 T4 several test cases have stick length all equal to 1, then I could have guessed more points... (Already been whipped by watermelon for an afternoon (just ten minutes))
3. (Metaphysics) Segment tree playing sugar songs before CSP finals caused RP to drain completely, fortunately I listened while eating chips, my RP was higher than segment tree who didn't have time to eat chips.

Sigh, everything belongs to the past, fight again next year.

Easter Eggs

Easter Egg 1: Passwords

1. Shang4Shan3Ruo6Shui4

2. Ren5Jie4Di4Ling3%
3. \1~i+iK29'EEG2hnG>fW?}CF~B-fSoXIE3_0=+PDM(.6ELR[^t
4. EA*y$1O}"D#Q4G[i?3L))aB+X"_1+$gU(!ZA!8<>3:D.aGjF

Easter Egg 2: My Code Written in the Exam Room

[My CSP 2025 Code.zip]

Easter Egg 3: Next Year's Travelogue

[CSP2026 Dive Writeup]

This article ends here, thank you all for reading.

Footnotes

1. Eating chips: CSP [return]
2. gjt: @[gaojt] [return]
3. whk: school subjects [return]
4. Pressure [return]
5. ABC: Atcoder Beginner Contest [return]
6. DP: DeepSeek [return]
7. fpnzlll: @[lll10] [return]
8. Watermelon: @[ouxiyao] [return]
9. [Data deleted] [return]
10. Segment tree: @[pk123456], named after his love for segment trees [return]
11. yty: @[Yonder] [return]
12. Just brute forces it: original words from yty [return]
13. Luudan (hard-boiled egg): @[ni_ju_ge] [return]