Given the length N of the string, we have to find the number of special strings of length N.
A string is called a special string if it consists only of lowercase letters a and b and there is at least one b between two a’s in the string. Since the number of strings may be very large, therefore print it modulo 10^9+7.
Input: N = 2
The number of special string so length 2 are 3 i.e. “ab”, “ba”, “bb”
Input: N = 3
The number of special string so length 3 are 5 i.e. “abb”, “aba”, “bab”, “bba”, “bbb”
To solve the problem mentioned above, the first observation is if the integer N is 0 then there can only be an empty string as the answer, if N is 1 then there can be two string “a” or “b” as an answer but if the value of N is greater than 1 then the answer is equal to the sum of previous two terms. Now to find the count of special strings we run a loop and for each integer i count of the special string of length i is equal to the sum of the count of special strings of length i-1 and count of special strings of length i-2. Store the value of each integer in an array and return the required answer.
Below is the implementation of the above approach:
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