Finding longest substring in alphabetical order

Question

EDIT: I am aware that a question with similar task was already asked in SO but I\'m interested to find out the problem in this specific piece of code. I am

Answer 1

I suppose this is problem set question for CS6.00.1x on EDX. Here is what I came up with.

s = raw_input("Enter the string: ")
longest_sub = ""
last_longest = ""
for i in range(len(s)):
    if len(last_longest) > 0:
        if last_longest[-1] <= s[i]:
            last_longest += s[i]
        else:
            last_longest = s[i]
    else:
        last_longest = s[i]
    if len(last_longest) > len(longest_sub):
        longest_sub = last_longest
print(longest_sub)