用Python 找最长子串

题目选自edX公开课 MITx: 6.00.1x Introduction to Computer Science and Programming 课程 Week2 的Problem Set 1的第三题。下面是原题内容。

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, ifs = '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

For problems such as these, do not include raw_input statements or define the variable s in any way. Our automated testing will provide a value of s for you - so the code you submit in the following box should assume s is already defined. If you are confused by this instruction, please review L4 Problems 10 and 11 before you begin this problem set.

代码如下：

#判断一个字符串内的字母是否是按字母表顺序
# 如IsStrIncre('abbcdg') 返回 True
# IsStrIncre('abbadg') 返回 False
# 如果只有一个字符，也返回False 
def IsStrIncre(s):
    for cnt in range(len(s) - 1):
        if len(s) == 1:
            return False        
        elif s[cnt] > s[cnt+1]:
            return False
    return True    

s = 'abajsiesnwdw'# example code

substr = ''

for length in range(1, len(s)+1):
    firstflag = True # a flag to remember the first string that satisfied the requirements
                     # and ignore the strings satisfied the requirements but appeared after
    for cnt in range(len(s)-length+1):        
        if IsStrIncre(s[cnt: cnt+length]):
           if firstflag: 
               substr = s[cnt: cnt+length]
               firstflag = False
        
print 'Longest substring in alphabetical order is: ' + substr