用Python求一个字符串中最长字典序不下降的子序列

今天上edX上的公开课的时候做到了这道题，觉得十分有趣。

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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”

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当然这其实只是个入门Python语言的题而已，不过毕竟是目前最难的一道，所以还是很兴奋的。

要点就是，Python里面的字符可以直接比较大小

顺便，按照这个课里面提交程序的惯例，s都是不定义的（测试的时候会加一句定义，类似于直接输入一个数据了吧）

prev = 'a'
fstar = True
cursn = 0
lonsn = 0
curstr = ''
lonstr = ''
for x in s:   
    if fstar == True or x >= prev:
        print(x,prev)
        fstar = False
        cursn += 1
        curstr += x
        if cursn > lonsn:
            lonsn = cursn
            lonstr = curstr
    else:
        fstar = False
        curstr = x
        cursn = 1
    prev = x
print('Longest substring in alphabetical order is: ',lonstr)