generates the number of valid strings for DFA in a particular homework problem I had

problem.py

def A(s, c):
    if len(s) <= c:
        return False

    if s[c] == '0':
        return B(s, c+1)
    elif s[c] == '1':
        return C(s, c+1)

def B(s, c):
    if len(s) <= c:
        return False

    if s[c] == '1':
        return D(s, c+1)
    else:
        return False

def C(s, c):
    if len(s) <= c:
        return False

    if s[c] == '0':
        return D(s, c+1)
    else:
        return False

def D(s, c):
    if len(s) <= c:
        return True

    if s[c] == '0':
        return A(s, c+1)
    elif s[c] == '1':
        return B(s, c+1)


def gencheck(k):
    # generate strings of len k
    
    count = 0
    
    for i in range(0, 2**k):
        s = bin(i)
        s = s[2:]

        if len(s) < k:
            s = ('0' * (k-len(s))) + s

        if A(s, 0):
            count += 1

    print('N(%d) = %d' % (k, count))


if __name__ == '__main__':
    for i in range(0, 20):
        gencheck(i)

results

N(0) = 0
N(1) = 0
N(2) = 2
N(3) = 0
N(4) = 2
N(5) = 4
N(6) = 2
N(7) = 8
N(8) = 10
N(9) = 12
N(10) = 26
N(11) = 32
N(12) = 50
N(13) = 84
N(14) = 114
N(15) = 184
N(16) = 282
N(17) = 412
N(18) = 650
N(19) = 976