codewar练习--1

题目：An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.翻译只包含不重复字母的连续字符串，忽略大小写

is_isogram("Dermatoglyphics") == true

is_isogram("aba") == false

is_isogram("moOse") == false# -- ignore letter case

---

def is_isogram(string):

#your code here

      string = string.lower()

      string_set = set(string)

      if len(string) != len(string_set):

            return False

      else:

            for i in string:

                  if i not in 'abcdefghijklmnopqrstuvwxyz':

                  return False

      return True