Modular Inverse

def extended_gcd(aa, bb):
    lastremainder, remainder = abs(aa), abs(bb)
    x, lastx, y, lasty = 0, 1, 1, 0
    while remainder:
        lastremainder, (quotient, remainder) = remainder, divmod(lastremainder, remainder)
        x, lastx = lastx - quotient * x, x
        y, lasty = lasty - quotient * y, y
    return lastremainder, lastx * (-1 if aa < 0 else 1), lasty * (-1 if bb < 0 else 1)


def modinv(a, m):
    g, x, y = extended_gcd(a, m)
    if g != 1:
        raise ValueError
    return x % m

# -----------------------

def modinv2(a, m):
    orgm = m
    x, lastx, y, lasty = 0, 1, 1, 0
    while m:
        a, (quotient, m) = m, divmod(a, m)
        x, lastx = lastx - quotient * x, x
        y, lasty = lasty - quotient * y, y
    if a != 1:
        raise ValueError
    return lastx % orgm