AtCoder Regular Contest 108
D
AB
AtCoder
⌀ 1987
原题
← CE →
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Problem Statement

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Given are an integer $N$ and four characters $c_{\mathrm{AA}}$, $c_{\mathrm{AB}}$, $c_{\mathrm{BA}}$, and $c_{\mathrm{BB}}$. Here, it is guaranteed that each of those four characters is A or B.

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Snuke has a string $s$, which is initially AB.

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Let $|s|$ denote the length of $s$. Snuke can do the four kinds of operations below zero or more times in any order:

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  1. Choose $i$ such that $1 \leq i < |s|$, $s_{i}$ = A, $s_{i+1}$ = A and insert $c_{\mathrm{AA}}$ between the $i$-th and $(i+1)$-th characters of $s$.
  2. Choose $i$ such that $1 \leq i < |s|$, $s_{i}$ = A, $s_{i+1}$ = B and insert $c_{\mathrm{AB}}$ between the $i$-th and $(i+1)$-th characters of $s$.
  3. Choose $i$ such that $1 \leq i < |s|$, $s_{i}$ = B, $s_{i+1}$ = A and insert $c_{\mathrm{BA}}$ between the $i$-th and $(i+1)$-th characters of $s$.
  4. Choose $i$ such that $1 \leq i < |s|$, $s_{i}$ = B, $s_{i+1}$ = B and insert $c_{\mathrm{BB}}$ between the $i$-th and $(i+1)$-th characters of $s$.
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Find the number, modulo $(10^9+7)$, of strings that can be $s$ when Snuke has done the operations so that the length of $s$ becomes $N$.

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Constraints

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  • $2 \leq N \leq 1000$
  • Each of $c_{\mathrm{AA}}$, $c_{\mathrm{AB}}$, $c_{\mathrm{BA}}$, and $c_{\mathrm{BB}}$ is A or B.
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Input

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Input is given from Standard Input in the following format:

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N
c_{\mathrm{AA}}
c_{\mathrm{AB}}
c_{\mathrm{BA}}
c_{\mathrm{BB}}

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Output

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Print the number, modulo $(10^9+7)$, of strings that can be $s$ when Snuke has done the operations so that the length of $s$ becomes $N$.

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样例

Input #1
4
A
B
B
A
Output #1
2
- There are two strings that can be $s$ when Snuke is done: `ABAB` and `ABBB`.
Input #2
1000
B
B
B
B
Output #2
1
- There is just one string that can be $s$ when Snuke is done.
题目信息
题号
arc108_d
时间限制
2 s
内存限制
1024 MB

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。