<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<= 2:
            return 0
        size = (n>>5)+1 
        bita = [0]*size

        def isnot(x):
            return (bita[x >> 5] & (1 << (x & 31))) != 0
        def setnot(x):
            bita[x >> 5] |= (1 << (x & 31))    



        for i in range(2, int(n**0.5)+1):
            if not isnot(i):
                for j in range(i*i,n,i):
                    setnot(j)

        cnt = 0
        for i in range(2,n):
            if not isnot(i):
                cnt+=1
        return cnt                    


        
18531