<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:
        if n <=1: return 0
        num = [False, False] + [True] * (n-2)
        for i in range(2, int(sqrt(n) + 1)):
            if num[i]:
                for k in range(i**2, n, i):
                    num[k] = False
        return sum(num)

        '''
        dic = set()
        cnt = 0
        for i in range(2, n):
            flag = True
            for p in dic:
                if p > i//2:
                    continue
                if i % p == 0:
                    flag = False
                    break
            if flag:
                dic.add(i)
                cnt +=1
        return cnt
        '''

                    


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