D - AB Editorial by evima
The answer is obviously \(1\) for \(N = 2, 3\). Below, we assume \(N>3\).
Let us consider the case \(c_{\mathrm{AB}} =\) B
. (The case \(c_{\mathrm{AB}} =\) A
is almost the same.)
If \(c_{\mathrm{BB}} =\) B
, there is only one string that can be made.
Below, we assume \(c_{\mathrm{BB}} =\) A
.
If \(c_{\mathrm{BA}} =\) A
here, we can construct every string that begins with AB
and ends with B
, so the answer is \(2^{N-3}\).
If \(c_{\mathrm{BA}} =\) B
, among the strings that begin with AB
and end with B
, we can construct everything that does not contain AA
as a substring. In this case, the answer is \(F_{N-2}\), where \(F\) is defined as follows:
\( F_{n} = \begin{cases} 1 & \text{if } n \leq 1 \cr F_{n-1} + F_{n-2} & \text{otherwise} \end{cases}\)
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