<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:
    def countPrimes(self, n: int) -> int:
        def sieve_of_eratosthenes(n): 
            is_prime = [True] * (n + 1) 
            if n >= 0: is_prime[0] = False
            if n >= 1: is_prime[1] = False 
            for i in range(2, int(n**0.5) + 1):
                if is_prime[i]:
                    for j in range(i * i, n + 1, i):
                        is_prime[j] = False
            return is_prime 
        prime_flags = sieve_of_eratosthenes(n)
         
        return len([i for i, prime in enumerate(prime_flags[:len(prime_flags)-1]) if prime]) 
108656