Leetcode: Count Primes

Count the number of prime numbers less than a non-negative number, n.

 1 public class Solution {
 2     public int countPrimes(int n) {
 3         if (n <= 1) return 0;
 4         boolean[] notPrime = new boolean[n+1];
 5         notPrime[0] = true;
 6         notPrime[1] = true;
 7         int count = 0;
 8         for (int i=2; i<n; i++) {
 9             if (!notPrime[i]) {
10                 count++;
11                 int mul = 2;
12                 while (i <= n/mul) {
13                     notPrime[i*mul] = true;
14                     mul++;
15                 }
16             }
17         }
18         return count;
19     }
20 }