def maxDigit(n: int) -> int:
    n = abs(n)
    if n == 0:
        return 0
    rest = n // 10
    ones = n % 10
    next = maxDigit(rest)
    return max(ones, next)

def testMaxDigit():
    print('Testing maxDigit()...', end='')
    assert(maxDigit(0) == 0)
    assert(maxDigit(1) == 1)
    assert(maxDigit(-1) == 1)
    assert(maxDigit(12321) == 3)
    assert(maxDigit(-12321) == 3)
    assert(maxDigit(102030405060708090) == 9)
    assert(maxDigit(22222222222) == 2)
    assert(maxDigit(22222322222) == 3)
    print('Passed!')

def main():
    testMaxDigit()

main()