AtCoder Grand Contest 040
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Problem Statement
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Given is a positive even number $N$.
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Find the number of strings $s$ of length $N$ consisting of A, B, and C that satisfy the following condition:
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$s$can be converted to the empty string by repeating the following operation:- Choose two consecutive characters in
$s$and erase them. However, choosingABorBAis not allowed.
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For example, ABBC satisfies the condition for $N=4$, because we can convert it as follows: ABBC → (erase BB) → AC → (erase AC) → (empty).
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The answer can be enormous, so compute the count modulo $998244353$.
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Constraints
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$2 \leq N \leq 10^7$$N$is an even number.
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Input
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Input is given from Standard Input in the following format:
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N
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Output
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Print the number of strings that satisfy the conditions, modulo $998244353$.
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样例
Input #1
2
Output #1
7
Except `AB` and `BA`, all possible strings satisfy the conditions.
Input #2
10
Output #2
50007
Input #3
1000000
Output #3
210055358
题目信息
题面与样例版权归 AtCoder 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。