<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:

        primes = [1] * (n - 2)  # 2 à n - 1
        ans = 0
        for k in range(2, math.floor(math.sqrt(n))+1):
            if primes[k - 2] == 0:
                continue
            ans += 1
            for i in range(k * k - 2, n - 2, k):
                primes[i] = 0
        
        return ans + sum(primes[math.floor(math.sqrt(n))-1:]) #  primes.count(1)

        """
        primes = [2, 3, 5, 7]
        for i in range(11, n, 2):
            if all(i%p for p in primes):
                primes.append(i)
        if n > 7:
            return len(primes)
        else:
            return len(list(filter(lambda x: x<n, primes)))
        """
91918