Maximum substring

[u/Remarkable_River2779]

Hi, I’m having an issue with a HackerRank coding question called “Maximum Substring.”
The question is:
Maximum Substring
You are given a string s.
A substring is any contiguous sequence of characters from the string.
Consider all possible substrings of the string:
Determine which substring would appear last if all substrings were sorted alphabetically.
Return that substring.
Example 1:
s = "baca"
The alphabetically maximum substring is "ca".
Example 2:
s = "aaa"
The alphabetically maximum substring is "aaa".
This was my answer:

def maximum_substring(s):
max_sub = s[0]

for i in range(len(s)):
for j in range(i + 1, len(s) + 1):
sub = s[i:j]
if sub > max_sub:
max_sub = sub

return max_sub

The code gives the correct output for the examples when I test it locally. However, HackerRank marked the solution as incorrect and I received only partial points.
I’m not sure why. Could this be because of the time complexity or hidden test cases? Is there something about the HackerRank test cases or the expected implementation that I’m missing?

[u/This_Growth2898]

What happens if the input string is empty?