Avoid the Set {00, 11}

Difficulty: 2400 4000 ms 128 MB

combinatorics aho-corasick matrix-exponentiation

Count the strings of length N over {0, 1} that contain none of 00, 11 as a substring, modulo 10⁹+7.

Input: one integer N.

Output: the count mod 10⁹+7.

Constraints:

• 1 ≤ N ≤ 10¹⁸

Sample Tests

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2

2

2

4

2

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