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

Here is a single pass solution with a fast loop. It reads each character only once. Inside the loop operations are limited to

• 1 string comparison (1 char x 1 char)
• 1 integer increment
• 2 integer subtractions
• 1 integer comparison
• 1 to 3 integer assignments
• 1 string assignment

No containers are used. No function calls are made. The empty string is handled without special-case code. All character codes, including chr(0), are properly handled. If there is a tie for the longest alphabetical substring, the function returns the first winning substring it encountered. Case is ignored for purposes of alphabetization, but case is preserved in the output substring.

def longest_alphabetical_substring(string):
    start, end = 0, 0     # range of current alphabetical string
    START, END = 0, 0     # range of longest alphabetical string yet found
    prev = chr(0)         # previous character

    for char in string.lower():   # scan string ignoring case
        if char < prev:       # is character out of alphabetical order?
            start = end       #     if so, start a new substring 
        end += 1              # either way, increment substring length 

        if end - start > END - START:  # found new longest?  
            START, END = start, end    #     if so, update longest 
        prev = char                    # remember previous character

    return string[START : END]   # return longest alphabetical substring 

Result

>>> longest_alphabetical_substring('drurotsxjehlwfwgygygxz')
'ehlw'
>>> longest_alphabetical_substring('eseoojlsuai')
'jlsu'
>>> longest_alphabetical_substring('hixwluvyhzzzdgd')
'luvy'
>>>