Given a string S, print the longest substring P such that P > S lexicographically.

longest_lex_substr.py

def longestSubstr(s):
    for i in range(1, len(s)):
        if s[i] > s[0]:
            break
    # For cases like, AABAB
    # BAB is lexicographically greater, however,
    # ABAB is the longest such substring
    while i > 1 and s[i-1] == s[0]:
        i -= 1

    return s[i:]
  
if __name__ == "__main__":
  print(longestSubstr("AABAB") == "ABAB")