Longest alphabetical substring - where to begin

Question

I am working on the "longest alphabetical substring" problem from the popular MIT course. I have read a lot of the information on SO about how to code it but I'm really struggling to make the leap conceptually. The finger exercises preceding it were not too hard. I was wondering if anyone knows of any material out there that would really break down the problem solving being employed in this problem. I've tried getting out a pen and paper and I just get lost. I see people employing "counters" of sorts, or strings that contain the "longest substring so far" and when I'm looking at someone else's solution I can understand what they've done with their code, but if I'm trying to synthesize something of my own it's just not clicking.

I even took a break from the course and tried learning via some other books, but I keep coming back to this problem and feel like I need to break through it. I guess what I'm struggling with is making the leap from knowing some Python syntax and tools to actually employing those tools organically for problem solving or "computing".

Before anyone points me towards it, I'm aware of the course materials that are aimed at helping out. I've seen some videos that one of the TAs made that are somewhat helpful but he doesn't really break this down. I feel like I need to pair program it with someone or like... sit in front of a whiteboard and have someone walk me step by step and answer every stupid question I would have.

For reference, the problem is as follows:

Assume s is a string of lower case characters.

Write a program that prints the longest substring of s in which the letters occur in alphabetical order. For example, if s = 'azcbobobegghakl', then your program should print

Longest substring in alphabetical order is: beggh

In the case of ties, print the first substring. For example, if s = 'abcbcd', then your program should print

Longest substring in alphabetical order is: abc

I know that it's helpful to post code but I don't have anything that isn't elsewhere on SO because, well, that's what I've been playing with in my IDE to see if I can understand what's going on. Again, not looking for code snippets - more some reading or resources that will expand upon the logic being employed in this problem. I'll post what I do have but it's not complete and it's as far as I get before I start feeling confused.

s = 'azcbobobegghakl'

current = s[0]
longest = s[0]

for letter in range(0, len(s) -1):
    if s[letter + 1] >= s[letter]:
        current.append(s[letter + 1])
        if len(current) > len(longest):
            longest = current
        else:
            current = 

Sorry for formatting errors, still new to this. I'm really frustrated with this problem.

Answer 1

You're almost there in your example, just needs a little tweaking

s = 'azcbobobegghakl'

longest = [s[0],]  # make them lists so we can manipulate them (unlike strings)
current = [s[0],]

for letter in range(0, len(s) -1):
    if s[letter + 1] >= s[letter]:
        current.append(s[letter + 1])
        if len(current) > len(longest):
            longest = current
    else:
        current = [s[letter+1],]  # reset current if we break an alphabetical chain

longest_string = ''.join(longest)  # turn out list back into a string

output of longest_string:

'beegh'