Find the maximum occurring character

Find the maximum occurring character in a given string.

def get_max_occuring_char(S):

    ASCII_SIZE = 256
    ctr = [0] * ASCII_SIZE
    max = -1
    ch = ''

    for c in S:
        ctr[ord(c)] += 1;

    for c in S:
        if max < ctr[ord(c)]:
            max = ctr[ord(c)]
            ch = c

    return ch

# test
print(get_max_occuring_char("Python: \
    Get file creation and modification date/times"))     # t
print(get_max_occuring_char("abcdefghijkb"))             # b

See also

https://www.w3resource.com/python-exercises/string/python-data-type-string-exercise-59.php