Project Euler: Problem 1 (Possible refactorings and run time optimizations)

Question

I have been hearing a lot about Project Euler so I thought I solve one of the problems in C#. The problem as stated on the website is as follows:

If w

Answer 1

Your approach is brute force apprach, The time complexity of the following approach is O(1), Here we     
are dividing the given (number-1) by 3, 5 and 15, and store in countNumOf3,countNumOf5, countNumOf15.
Now we can say that 3 will make AP, within the range of given (number-1) with difference of 3. 
suppose you are given number is 16, then 
3=> 3, 6, 9, 12, 15= sum1=>45
5=> 5, 10, 15  sum2=> 30
15=> 15 =>   sum3=15  
Add sum= sum1 and sum2


Here 15 is multiple of 3 and 5  so remove sum3 form sum, this will be your answer. **sum=sum-    
sum3** please check link of my solution on http://ideone.com/beXsam]

import java.util.*;
class Multiplesof3And5 {        
        public static void main(String [] args){
            Scanner scan=new Scanner(System.in);
            int num=scan.nextInt();
            System.out.println(getSum(num));
        }    
        public static  long getSum(int n){
            int countNumOf3=(n-1)/3;//             
            int countNumOf5=(n-1)/5;           
            int countNumOf15=(n-1)/15;            
            long sum=0;           
            sum=sumOfAP(3,countNumOf3,3)+sumOfAP(5,countNumOf5,5)-sumOfAP(15,countNumOf15,15);
            return sum;
        }
        public static int sumOfAP(int a, int n, int d){
            return (n*(2*a +(n -1)*d))/2;
        }
}