Class Assignment problem

I am getting wrong answers in test cases. Kindly let me know where my code went wrong. Here is my code:

@Puneet_Sharma,

Suggested approach:

If observed carefully , we can identify that this is a problem for fibonacci series. This is because at the nth place , there are two possibilities.

Possibility 1 : We can choose to place the current character as β€˜a’. If so , then it doesn’t matter whether we placed β€˜a’ or β€˜b’ at the previous position. The total number of ways in this possibility would equal to f(n-1)

Possibility 2 : We can place the current character as β€˜b’. However we can only do it if the previous character was not β€˜b’ . Hence the total number of ways for this case must be f(n-2)

We add these two possibilities up and obtain the recursive relation
f(n) = f(n-1) + f(n-2)
This is clearly the recursive relation for Fibonacci Series .