objective-c获得一个string的最后2个字符?

说我有一个string:

NSString *state = @"California, CA"; 

有人可以告诉我如何从这个string中提取最后两个字符(在这个例子中是@“CA”)

 NSString *code = [state substringFromIndex: [state length] - 2]; 

应该这样做

Swift 4:

 let code = (state as? NSString)?.substring(from: state.characters.count - 2)