本文實例講述了Python實現(xiàn)簡單查找最長子串功能。分享給大家供大家參考，具體如下：

題目選自edX公開課 MITx: 6.00.1x Introduction to Computer Science and Programming 課程 Week2 的Problem Set 1的第三題。下面是原題內容。

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.

代碼如下：

# -*- coding:utf-8 -*-
#! python2
#判斷一個字符串內的字母是否是按字母表順序
# 如IsStrIncre('abbcdg') 返回 True
# IsStrIncre('abbadg') 返回 False
# 如果只有一個字符，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

運行結果：

Longest substring in alphabetical order is: ajs

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