There is shop of items, users buying them.
Find k most popular items
Implement datastructure to hold items and 2 methods :
addItem(Item n)
getMostPopular(int k)
what is complexity of each one.

class Item{
	String id ;
	String name;
}


class DataStructure{
	void addItem(Item n) {
	  //implement this
	}
	
	ArrayList<Item> getMostPopular(int k){
	//implement this
	}
}