Leetcode 204: Count Primes

Description:

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

 

public class Solution {
    public int CountPrimes(int n) {
        var notPrime = new bool[n + 1];
        int count = 0;
        
        for (int i = 2; i < n; i++)
        {
            if (!notPrime[i])
            {
                count++;
            }
            
            for (int j = 2; i * j <= n; j++)
            {
                notPrime[i * j] = true;
            }
        }
        
        return count;
    }
}