编写一个程序，打印 s 中字母按字母顺序出现的最长子串。例如，如果s = 'azcbobobegghakl'，那么你的程序应该按字母顺序打印最长的子串是：beggh。
在平局的情况下，打印第一个子字符串。例如，如果 s = 'abcbc
s = "azcbobobegghakl"
x = s[0]
y = s[0]
for i in range (1, len(s)):
if s[i] >= s[i-1]:
y += s[i]
else:
y = s[i]
if len(y) > len(x):
x = y
print(x)
【问题讨论】：
标签： python string python-3.x for-loop
这闻起来像家庭作业，但是... 下面是cmets的解释：
# assign a string to a variable named s
s = "azcbobobegghakl"
# assign the zeroth character of the string s to x
x = s[0]
# assign the zeroth character of the string s to y
y = s[0]
# loop through a range of numbers starting at 1 and going to the length of s
# within each loop, the variable i tells us which iteration of the loop
# we're currently in.
for i in range(1, len(s)):
# compare the character in s at the position equal
# to the current iteration number to see if it's greater
# than or equal to the one before it. Alphabetic characters
# compared like this will evaluate as numbers corresponding
# to their position in the alphabet.
if s[i] >= s[i-1]:
# when characters are in alphabetical order, add them to string y
y += s[i]
else:
# when the characters are not in alphabetical order, replace y with
# the current character
y = s[i]
# when the length of y is greater than of x, assign y to x
if len(y) > len(x):
x = y
# after finishing the loop, print x
print(x)
【讨论】：
python 中的string 类包含__lt__、__eq__ 数据模型方法，这使我们能够做到-
str1 = 'aaa'
str2 = 'bbb'
str3 = 'aaa'
assert str2 < str1 # Will lead to AssertionError
# '<' calls the __lt__() method of the string class
assert str1 == str3 # No AssertionError
#'==' calls the __eq__() method of the string class
string 类中的这些特定数据模型方法比较字符串中每个字符的 ASCII 值。英文字母表中每个字符的 ASCII 值依次递增，即 'A'
您的代码
您一次遍历字符串一个字符（从第二个字符开始），并检查当前字符的 ASCII 值是否大于（或等于）前一个字符。如果是，则将该字符添加到y 字符串中，并将生成的字符串存储为y。如果不是，则将 y 替换为当前字符。最后，如果y 的字符多于x，则将x 中的字符串替换为y。
【讨论】：