OBJC Tips

OBJC Tips

* strong, weak, assign
http://stackoverflow.com/questions/8927727/objective-c-arc-strong-vs-retain-and-weak-vs-assign

* bridge (void* => id, id => void*)
http://stackoverflow.com/questions/18976016/how-to-specify-property-to-c-pointer-in-arc-environment

* arr.indexOf
```Objective-C
//BAD: it returns max int.
(int)[arr indexOfObject:obj]>0

//Good
[arr indexOfObject:obj]==NSNotFound

//Better
[arr containsObject:obj]
```

* str format
use "%llu", for long long

* use long long instead of long