Print the longest alphabetical substring using Python and for ties, print the first substring [duplicate]

问题

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

Here's the code I found. How do I implement the latter condition in the question given above regarding ties?

    *s = raw_input('provide string: ')
    result = []
    final = []
    for letters in s:
        result = result + [letters]        
        if result == sorted(result) and len(result) >= len(final):
            final = result            
        elif result != sorted(result):
            result = [result[len(result)-1]]        
    print('Longest substring in alphabetical order is: '+(''.join(final)))*

回答1:

I'd approach the problem the following way:

Let's define two strings: The current string of increasing letters and the currently longest string.
Both strings are initialized with the first letter. (This way we can always read their last letter.)
Then we iterate over the input string s (starting with the second character).
If the current character c fulfills the requirement c >= current[-1], we add it to the current solution.
We possibly store the current string as longest.
If c does not fulfill the ordering requirement, we start with a new solution current = c.
Finally, we print the longest string.

s = "azcbobobegghakl"
longest = s[0]
current = s[0]
for c in s[1:]:
    if c >= current[-1]:
        current += c
        if len(current) > len(longest):
            longest = current
    else:
        current = c
print "Longest substring in alphabetical order is:", longest

How to fix your code wrt. the mentioned condition:

Use > instead of >= in the comparison len(result) >= len(final), i.e. only update the final solution if it is longer, but not if it has the same length.

Considering Dylans comment

You're right. I updated both code and description to correctly handle the case when s ends with the longest alphabetical substring. (Moving else: two lines down was sufficient.)

来源：https://stackoverflow.com/questions/31065413/print-the-longest-alphabetical-substring-using-python-and-for-ties-print-the-fi

标签

python

python-2.7

substring

alphabetical