CountPrimes.java

package math;

import java.util.BitSet;

/**
 * Created by gouthamvidyapradhan on 21/03/2017. Description:
 *
 * <p>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 = (int) Math.sqrt(n);
    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;
  }
}