11-字符串常见操作(find,index,count,replace,split)
12-字符串常见操作(capitalize,title,startswith,endswith,lower,upper)
13-字符串常见操作(ljust,rjust,center,lstrip,rstrip,strip)
14-字符串常见操作(rfind,rindex,partition,rpartition,splitlines)
15-字符串常见操作(isalpha,isdigit,isalnum,isspace,join)