34 lines
621 B
Markdown
34 lines
621 B
Markdown
Your librarian has a 2-row bookshelf that can contain N books in each row. She wants to know the number of ways that she can fill the bookshelf with red-colored books and blue-colored books such that no 2 red-colored books are adjacent to each other (horizontally or vertically).
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Input: the integer, N (1<=N<=2^1024)
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Output: the number of ways you can place red-colored books and blue-colored books onto a N-column bookshelf. Since this number might be really big, output it mod 10^9+7.
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Example:
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Input: 2
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Your valid bookshelf layouts are:
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```
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BB
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BB
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BB
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BR
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BR
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BB
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RB
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BB
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BB
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RB
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RB
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BR
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BR
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RB
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```
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Therefore,
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Output: 7
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