python实现:最长子字符串

给定一个字符串 s 和正整数 n，
请使用你熟悉的编程语言输出 s 中包含不超过 n 种字符的最长子串，
如 s="uabbcadbaef"，n=4 时应该输出 "abbcadba"。

 # 判断一个字符串里面有几个不同字目
 def count_diff(s):
     arr = []
     for i in s:
         if i not in arr:
             arr.append(i)
     return len(arr)

 # 获得最长子串
 def get_longest_str( s, n ):
     res_temp = ''
     length = len(s)
     for i in range(length-n):
         for j in range(i,length):
             str_temp = s[i:j]
             if count_diff( str_temp )<=n and  len(str_temp)>len(res_temp):
                 res_temp = str_temp
     return res_temp

 if __name__ == '__main__':
     s = "uabbcadbaef"
     n = 4
     res = get_longest_str( s,n )
     print( res )