<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 SieveOfEratosthenes(num: int):
            prime = [True for _ in range(num+1)]
            prime[0] = prime[1] = False
            p = 2
            while (p * p <= num):
                if (prime[p] == True):
                     for i in range(p * p, num+1, p):
                        prime[i] = False
                p += 1
            res = []
            for i in range(len(prime)):
                if prime[i]:
                    res.append(i)
            return prime
        if n == 0:
            return 0
        sie = SieveOfEratosthenes(n)[:n]
        ans = 0
        for v in sie:
            ans += 1 if v else 0
        return ans
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