Count Primes

https://www.lintcode.com/problem/count-primes/description

public class Solution {
    /**
     * @param n: a integer
     * @return: return a integer
     */
    public int countPrimes(int n) {
        // write your code here
        int res = 0;
        for (int i = 2; i < n; i++) {
            if (isPrime(i)) {
                res++;
            }
        }
        return res;
    }

    private boolean isPrime(int i) {
        for (int j = 2; j <= i / j; j++) {
            if (i % j == 0) {
                return false;
            }
        }
        return true;
    }
}