Question

Let’s say I have a List of Employee Objects. The Employee Objects have a getDepartment method which returns a Department Object. I want to iterate through that list to find the department with the most Employees (i.e. the Department Object returned most often from getDepartment). What is the fastest way to do this?

public class Employee{

   static allEmployees = new ArrayList<Employee>();       

   int id;
   Department department;

   public Employee(int id, Department department){
     this.id = id;
     this.department = department;
     allEmployees.add(this);
   }

   public Department getDepartment(){
     return department;
   }

   public static List<Employee> getAllEmployees(){
      return allEmployees;
   }
}

public class Department{
   int id;
   String name;

   public Department(int id){
     this.id = id;
   }

   public String getName(){
     return name;
   }
}

If there’s two departments with an equal number of employees it doesn’t matter which is returned.

Thanks!

Answer 1

create a map of department id -> counts.

that way you get a collection of all the counts by id. You can also maintain a max item that is a reference to the map entry with the highest count.

the algorithm would be something like:

1) Initialize a Map and a currentMax
2) loop through employees
3) for each employee, get its department id
4) do something like map.get(currentId)
a) if the current count is null, initialize it
5) increment the count
6) if the incremented count is > currentMax, update currentMax

This algorithm will run in O(n); I don’t think you can get any better than that. Its space complexity is also O(n) because the number of counts is proportional to the size of the input.

If you wanted, you could create a class that uses composition (i.e. contains a Map and a List) and also manages keeping pointers to the Entry with the highest count. That way this part of your functionality is properly encapsulated. The stronger benefit of this approach is it allows you to maintain the count as you enter items into the list (you would proxy the methods that add employees to the list so they update the map counter). Might be overkill though.