public int countPrimes(int n) {

     ArrayList<Integer> primes = new ArrayList<Integer>();

     if (n <= ) return Math.max(, n - );

     primes.add();

     primes.add();

     for (int i = ; i <= n; i++) {

         boolean isPrime = true;

         for (int p : primes) {

             int m = i % p;

             if (m == ) {

                 isPrime = false;

                 break;

             }

         }

         if (isPrime) {

             primes.add(i);

         }

     }

     return primes.size();

 }

 class Solution {

     public int countPrimes(int n) {

         boolean[] notPrime = new boolean[n];

         int count = ;

         for (int i = ; i < n; i++) {

             if (notPrime[i] == false) {

                 count++;

                 for (int j = ; i * j < n; j++) {

                     notPrime[i * j] = true;

                 }

             }

         }

         return count;

     }

 }