s = 'aabbccabaddddddabcccccddd'


# s = xxxxxxxxxxxxxAAAAAAAAAxxxxxxxxxxxxxxxxx
#                  i        j

i = 0

longest = ''

while i < len(s):
    j = i + 1
    while j < len(s) and s[j] == s[j - 1]:
        j += 1
    # print(s[i:j])

    if j - i > len(longest):
        longest = s[i:j]

    i = j

print('den længste blok af identiske tegn er', longest)