CountPrimes.java

package math;

import java.util.BitSet;

/**
 * Created by gouthamvidyapradhan on 21/03/2017.
 Description:

 Count the number of prime numbers less than a non-negative number, n.
 */
public class CountPrimes
{
    /**
     * Main method
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception
    {
        System.out.println(new CountPrimes().countPrimes(999187));
    }

    public int countPrimes(int n)
    {
        if(n == 0 || n == 1 || n == 2) return 0;
        else if(n == 3) return 1;
        BitSet set = new BitSet();
        n = n - 1;
        int sqRt = new Double(Math.sqrt(n)).intValue();
        int count = n;
        for(int i = 2; i <= sqRt; i ++)
        {
            if(!set.get(i))
            {
                for(int j = 2; (i * j) <= n; j ++)
                {
                    if(!set.get(i * j))
                    {
                        count --;
                        set.set(i * j);
                    }
                }
            }
        }
        return count - 1;
    }

}