<p>Given an integer <code>n</code>, return <em>the number of prime numbers that are strictly less than</em> <code>n</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 10
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 0
<strong>Output:</strong> 0
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= n &lt;= 5 * 10<sup>6</sup></code></li>
</ul>

class Solution {
public:
    int countPrimes(int n) {
        vector<int> primes;

        int i = 2; 
        while(i < n) {
            if(i==2) { primes.push_back(2); i++; continue; } 
            if(i==3) { primes.push_back(3); i+=2; continue; } 

            int j = 1;
            bool is_prime = true;
            while(primes[j]*primes[j] <= i) {
                if(i%primes[j]==0) { is_prime = false; break; }
                j++;
            }
            if(is_prime) { primes.push_back(i); }
            i+=2;
        }

        return primes.size();
    }
};
34500