What are collection types in objective C and Swift?

Ans : 

In  iOS, there are 3 collection types. 

i. Array 
ii. Dictionary 
iii. Set.

There are two types of collection types. Mutable and Immutable. Mutable can be changed but immutable can not be changed once initialised.

Q. How to check nil keys in dictionary?
A. There is not any nil keys in dictionary.