Count Primes

 1 public class Solution {
 2     public int countPrimes(int n) {
 3 
 4         if(n < 2) return 0;
 5 
 6         BitSet b = new BitSet();
 7 
 8         b.set(0);
 9         b.set(1);
10 
11         for(int p = 2; p * 2 < n ; p = b.nextClearBit(p + 1)){
12             for(int i = 2; p * i < n ;i++){
13                 b.set(p * i);
14             }
15         }
16 
17         b.flip(0, n);
18 
19         return b.cardinality();
20     }
21 }