204. Count Primes

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

Example:

Input: 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

int countPrimes(int n) {
    if (n <= 1) return 0;
    vector<bool> primes(n, true);
    int count = 0;
    primes[0] = primes[1] = false;
    for (int i = 2; i < n; ++i) {
        if (!primes[i]) continue;
        ++count;
        for (int j = 2; i * j < n; ++j) {
            primes[i * j] = false;
        }
    }
    return count;
}

int countPrimes(int n) {
    if (n <= 1) return 0;
    vector<bool> primes(n, true);
    int count = 0;
    primes[0] = primes[1] = false;
    for (int i = 2; i * i < n; ++i) {
        if (!primes[i]) continue;
        for (int j = i * i; j < n; j += i) {
            primes[j] = false;
        }
    }
    for (bool prime : primes) {
        if (prime) ++count;
    }
    return count;
}