Leader

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It is a great honor for me to represent China at IOI 2024 held in Egypt. During the period when the national team was selected to participate in IOI, we participated in a total of six training sessions arranged by CCF, which maintained our competitive state and improved our level.

After a flight of more than ten hours, we arrived in Egypt. The time difference between Egypt and China is five hours, which didn't affect me much. Our competition was held in a school in Egypt. Although the school environment was quite nice, the situation outside the campus was quite different. The area around the school seemed to be a backward rural area. We walked around but couldn't find a restaurant. Fortunately, the food at the school was acceptable, so we didn't have the need to eat out. It's still better at home.

The keyboards provided in the competition venue are different from the ones commonly used in China, so we need to purchase our own keyboards and bring them into the examination room during the trial run (so that they can check the keyboards after the trial run). During the trial run, I passed all four questions and tested the efficiency of the competition website. I found that the competition evaluation machine was actually very slow, with a speed of only about two-thirds of that of my own machine.

The strategy I set in formal competitions is to solve each question one by one. If I spend a lot of time on one question, I will move on to the next one.

In the first competition, I started with the first problem, Niles. This problem was relatively simple and I passed it around 35 minutes. The second problem, Message, was a communication problem. I found that I could get all the good positions by having each position pass on a good position to the next one. However, since I thought this approach would require four rounds of transmission, I believed it wouldn't pass. After some time and still having no other ideas, I decided to implement this approach first. During the implementation, I realized I had missed a condition in the problem: Alice could know what information Bob received. I thought about the use of this condition and found that Bob could do some interactions between "good guys" and "bad guys". This idea led to a promising 90-point solution, which I implemented and then moved on to the third problem, Tree. The process of solving Tree went smoothly. I found that for single queries, I could use greedy or dynamic programming, and for multiple queries, I could split the contributions. In the end, only a union-find set was needed. I passed this problem in 40 minutes. Then I went back to think about the second problem and found that adding one more line of code could make it a full score solution. So I passed the second problem around 3 hours into the competition.

After the competition, I found out that I was the only one who got a perfect score. We exchanged solutions to the problems with the leader of the Bulgarian team. I felt that this leader was very competent and was a rare coach who could solve difficult problems. In the afternoon, we also participated in the card game event organized by Jane Street, which was very interesting. In the evening, our leader, Mr. Han Wentao, held a short meeting for us, summarizing today's competition and reminding everyone that the problems were arranged in alphabetical order, not by difficulty.

We participated in a beach activity within two days. As everyone had not been in the water for a long time, we decided not to go into the sea for safety reasons and instead played mahjong on the beach. For lunch, we ate pizza at a restaurant near the beach. The pizza was enough for the five of us and only cost about 50 yuan. The prices in Egypt are really low.

On the second day of the competition, I still chose to start with the first problem. The problem "Hieroglyphs" was straightforward and seemed not too difficult. It was divided into two steps: finding a unique possible answer and making a judgment. I guessed that the first step could be solved by greedy algorithm and the second step by dynamic programming. However, when testing the sample cases, I found that my first step was incorrect, but I didn't know how to fix it. By then, an hour had passed, so I decided to skip the first problem for the time being. The second problem "Mosaic" was equivalent to a classic game model, and I passed it in 15 minutes. The third problem "Sphinx" was an interactive problem. I realized that I could do binary search after odd-even coloring of the DFS tree, which would result in 2n + 2nlogn interactive times; and if I merged adjacent points of the same color in advance, the interactive times could be optimized to 3n + nlogn. I passed this problem around 3 hours into the competition. Looking back at the first problem, I quickly discovered a conclusion that I hadn't noticed before: the occurrence times of each character in the answer must be the smaller value of its occurrence times in the two strings. Following this conclusion, I found that the answer was the merge of the two sequences, and the merge order could also be greedy, thus solving the first part of this problem. I submitted this part and got the score of subtask 4. Although the dynamic programming part was relatively simple, there were many details. After some debugging, I passed this problem at 4 hours and 20 minutes.

The closing ceremony was held in front of the pyramids, with the Sphinx also standing there. The dinner provided at the closing ceremony was very high-end, and the speeches given by the guests on stage were all very meaningful and inspiring to me. I got my championship trophy as I wished.

Beyond the competition, the IOI is also an excellent opportunity for international exchange. During the IOI, we saw the magnificent pyramids and exquisite Egyptian sculptures, and learned about some local cultures. We also met contestants from various regions and countries around the world, including Hong Kong, the United States, Canada, South Korea, North Korea, Australia, and more. One foreign contestant told us that he was doing the training team mutual testing on QOJ and exchanged with us about the training and selection process of OI; the Australian contestants secretly pinned koala dolls on everyone's clothes on the way to the opening ceremony; and there were many Chinese contestants who could communicate with us directly in Chinese.

Finally, I would like to express my gratitude to CCF for providing us with the opportunity to compete abroad. I am also thankful to my parents and classmates for their care and support. I am grateful to teachers Xu Xianyou, Wang Jiahong and Zhou Bang for their guidance. I would also like to thank the teachers who accompanied us for their concern and care along the way.

LJKGS6789 2024-09-25 15:28:16