Count Primes

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

public int countPrimes(int n) {

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

    if (n <= 3) return Math.max(0, n - 1);

    primes.add(2);
    primes.add(3);

    for (int i = 4; i <= n; i++) {
        boolean isPrime = true;
        for (int p : primes) {
            int m = i % p;
            if (m == 0) {
                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 = 0;
        for (int i = 2; i < n; i++) {
            if (notPrime[i] == false) {
                count++;
                for (int j = 2; i * j < n; j++) {
                    notPrime[i * j] = true;
                }
            }
        }
        return count;
    }
}