def remove_duplicates(strng): """ Returns a string which is the same as the argument except only the first occurrence of each letter is present. Upper and lower case letters are treated as different. Only duplicate letters are removed, other characters such as spaces or numbers are not changed >>> remove_duplicates("Mississippi") 'Misp' >>> remove_duplicates("apple") 'aple' >>> remove_duplicates("The quick brown fox jumps over the lazy dog") 'The quck borwn fx jmps v t lazy dg' >>> remove_duplicates("121 balloons 2 u") '121 balons 2 u' """ 可以通过测试 如果只用到最基础的loop 像 for ,while . join method等 如何做 不用set 和.isalpha