python - 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 also aware that this problem can be solved without using recursion.

The task is to write a program which will find (and print) the longest sub-string in which the letters occur in alphabetical order. If more than 1 equally long sequences were found, then the first one should be printed. For example, the output for a string abczabcd will be abcz.

I have solved this problem with recursion which seemed to pass my manual tests. However when I run an automated tests set which generate random strings, I have noticed that in some cases, the output is incorrect. For example:

if s = 'hixwluvyhzzzdgd', the output is hix instead of luvy

if s = 'eseoojlsuai', the output is eoo instead of jlsu

if s = 'drurotsxjehlwfwgygygxz', the output is dru instead of ehlw

After some time struggling, I couldn't figure out what is so special about these strings that causes the bug.

This is my code:

pos = 0
maxLen = 0
startPos = 0
endPos = 0


def last_pos(pos):
    if pos < (len(s) - 1):
        if s[pos + 1] >= s[pos]:
            pos += 1
            if pos == len(s)-1:
                return len(s)
            else:
                return last_pos(pos)
        return pos


for i in range(len(s)):
    if last_pos(i+1) != None:
        diff = last_pos(i) - i
    if diff - 1 > maxLen:
        maxLen = diff
        startPos = i
        endPos = startPos + diff

print s[startPos:endPos+1]

Answer 1

There are many things to improve in your code but making minimum changes so as to make it work. The problem is you should have if last_pos(i) != None: in your for loop (i instead of i+1) and you should compare diff (not diff - 1) against maxLen. Please read other answers to learn how to do it better.

for i in range(len(s)):
    if last_pos(i) != None:
        diff = last_pos(i) - i + 1
    if diff > maxLen:
        maxLen = diff
        startPos = i
        endPos = startPos + diff - 1