在给定字符串中按字母顺序查找最长的字母子串 [英] finding the longest substring of letters in alphabetical order in a given string

问题描述

当为给定的任务执行我的代码时，我不断得到最长的字符串加上迭代中的下一个字母.例如，如果我使用

s = 'azcbobobegghakl'

当答案应该是 "beggh" 时，我会得到 "beggha" 作为最长的字符串.我尝试过的所有随机字母字符串都会发生同样的错误.

我发现在result += letters"语句之后添加了额外的字母，但我不知道如何修复它.这是我的代码:

s = 'azcbobobegghakl'结果 = []最终 = []对于 s 中的字母:结果 += 字母如果 result == sorted(result) 和 len(result) >= len(final):最终=结果elif 结果 != 排序(结果):结果 = [结果[len(result)-1]]打印 "".join(final)

解决方案

这里的问题是 result 和 final 指向同一个列表.您可能认为 += 会在您发出 result += 字母 时创建一个新列表，但它不会:

<预><代码>>>>x = [1,2]>>>y = x>>>x += [3]>>>X[1, 2, 3]>>>是[1, 2, 3]>>>x 是 y真的

但是，当您使用 x = x + [3] 时:

<预><代码>>>>x = [1,2]>>>y = x>>>x = x + [3]>>>X[1, 2, 3]>>>是[1, 2]>>>x 是 y错误的

有关此行为的说明，请参阅此问题.当 letters 是字符串中的最后一个 a 字符时，这就是 for 循环(原始代码的)中发生的情况:

1. 开头，final 和 result 都指向 ['b', 'e', 'g', 'g', 'h'].
2. after result += 'a' final 和 result 都指向 ['b', 'e', 'g', 'g', 'h', 'a'].
3. 现在输入了 elif 块，结果将指向一个新列表 ['a']，而 final 仍然指向 <代码>['b', 'e', 'g', 'g', 'h', 'a'].
4. final 在此之后将不再更新

因此，您的原始代码(在编辑之前)可以通过更改来修复

result += 字母

到

result = result + [字母]:

s = 'azcbobobegghakl'结果 = []最终 = []对于 s 中的字母:结果 = 结果 + [字母]如果 result == sorted(result) 和 len(result) >= len(final):最终=结果elif 结果 != 排序(结果):结果 = [结果[len(result)-1]]打印(最终)

When executing my code for the given task, I keep getting the longest string plus the next letter in the iteration. For example, if I use

s = 'azcbobobegghakl' 

I will get "beggha" as the longest string, when the answer is supposed to be "beggh". This same bug occurs for all strings of random letters I tried.

I have figured out that the extra letter gets added on after the "result += letters" statement, but I'm not sure how to fix it. Here is my code:

s = 'azcbobobegghakl'
result = []
final = []
for letters in s:
    result += letters
    if result == sorted(result) and len(result) >= len(final):
        final=result
    elif result != sorted(result): 
        result = [result[len(result)-1]]
print "".join(final)

解决方案

The problem here is that result and final point to the same list. You are probably thinking that += will create a new list when you issue result += letters, but it won't:

>>> x = [1,2]
>>> y = x
>>> x += [3]
>>> x
[1, 2, 3]
>>> y
[1, 2, 3]
>>> x is y
True

However, when you use x = x + [3]:

>>> x = [1,2]
>>> y = x
>>> x = x + [3]
>>> x
[1, 2, 3]
>>> y
[1, 2]
>>> x is y
False

For an explanation of this behavior, see this question. This is what's happening in your for loop (edit: of your original code) when letters is the last a character in your string:

1. at the beginning, final and result both point to ['b', 'e', 'g', 'g', 'h'].
2. after result += 'a' final and result both point to ['b', 'e', 'g', 'g', 'h', 'a'].
3. now the elif block is entered and result will point to a new list ['a'], while final still points to ['b', 'e', 'g', 'g', 'h', 'a'].
4. final will never be updated again after this

Hence, your original code (before you edited it) can be fixed by changing

result += letters

to

result = result + [letters]:

s = 'azcbobobegghakl'
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(final)

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