NSString *test = @"test";
NSString *testUp = [test uppercaseString]; //大写
NSString *testUpFirst = [test capitalizedString]; //开头大写,其余小写
NSString *TEACHER =@"TEACHER";
NSString *TEACHERLower = [TEACHER lowercaseString]; //小写
NSString *TEACHERUpFirst = [TEACHER capitalizedString]; //开头大写,其余小写