edX MITx: 6.00.1x Introduction to Computer Science and Programming Using Python 课程 Week 1: Python Basics Problem Set 1 Problem 3

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

# Paste your code into this box
count = 1
result = s[0]
while s:
newcount = 1
newresult = ''
i = 0
while i+1<len(s):
if ord(s[i]) <= ord(s[i+1]):
newcount+=1
newresult+=s[i+1]
else:
break
i+=1
if newcount>count:
count = newcount
result = s[0]+newresult
s = s[i+1:]
print(result)

注：因为只是课程前期，故未使用sort()函数或一些其他高级函数。

有几点需要注意的地方：

1）不可去掉 newcount, newresult 变量，因为要找s中的最长子串，故如果后面能找到要替换掉前面稍短的子串

2）ord(s[i]) <= ord(s[i+1]):     %注意是小于等于号

3）s = s[i+1:]　　　　　　　 %注意i+1:后面不加空格