AtCoder Beginner Contest 122
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Problem Statement
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You are given an integer $N$. Find the number of strings of length $N$ that satisfy the following conditions, modulo $10^9+7$:
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- The string does not contain characters other than
A,C,GandT. - The string does not contain
AGCas a substring. - The condition above cannot be violated by swapping two adjacent characters once.
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Note
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A substring of a string $T$ is a string obtained by removing zero or more characters from the beginning and the end of $T$.
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For example, the substrings of ATCODER include TCO, AT, CODER, ATCODER and . (the empty string), but not AC
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Constraints
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$3 \leq N \leq 100$
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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 of length $N$ that satisfy the following conditions, modulo $10^9+7$.
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样例
Input #1
3
Output #1
61
There are $4^3 = 64$ strings of length $3$ that do not contain characters other than `A`, `C`, `G` and `T`. Among them, only `AGC`, `ACG` and `GAC` violate the condition, so the answer is $64 - 3 = 61$.
Input #2
4
Output #2
230
Input #3
100
Output #3
388130742
Be sure to print the number of strings modulo $10^9+7$.
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。