Ternary Strings Avoiding 22
combinatorics matrix-exponentiation
Count the strings of length N over {0, 1, 2} that never contain two consecutive 2s, modulo 109+7.
Input: one integer N.
Output: the count modulo 109+7.
Constraints:
- 1 ≤ N ≤ 1018
Sample Tests
Input 1
1
Output 1
3
Input 2
2
Output 2
8
Input 3
3
Output 3
22
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