How to get longest alphabetically ordered substring in python

I am trying to write a function that returns the longest substring of s in which the letters occur in alphabetical order. For example, if s = 'azcbobobegghakl', the function should return 'beggh'

Here is my function, which is still not complete but it does not return the list of sub; the return error is:

"IndexError: string index out of range"

def longest_substring(s):
    sub=[]
    for i in range (len(s)-1):
        subs=s[i]
        counter=i+1
        while ord(s[i])<ord(s[counter]):
            subs+=s[counter] 
            counter+=1
        sub.append(subs)
    return sub

It is not optimal (works in linear time O(n)) but i made some modification to your code (in Python 3):

def longest_substring(s):
    length = len(s)
    if length == 0 :         # Empty string
        return s
    final = s[0]
    for i in range (length-1):
        current = s[i]
        counter = i+1
        while counter < length and ord(s[i]) <= ord(s[counter]):  
            current += s[counter] 
            counter +=1
            i+=1
        if len(final) < len(current):
            final = current
    return final
s = 'azcbobobegghakl'
print(longest_substring(s))

Output:

beggh

Modifications:

• You are comparing character with fixed position i.e. in while loop you are incrementing only counter not i so I incremented the ith position also.(So we avoid checking the characters which are already checked, So it does this in linear time O(n) I think..)
• Also you are only checking less than for condition while ord(s[i])<ord(s[counter]): But you also have to check for equals too.
• You created one list where you append every sequence which is unnecessary unless you want do any other calculations on the sequence, So I take string and if previous sequence's length is small then I updated it with new sequence.

Note : If two sequence's length is same then 1st occurring sequence is shown as output.

Another Input:

s = 'acdb'

Output:

acd

I hope this will help you.