<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 isprime(n):
        #     if n<=1:
        #         return False
            
        #     for x in range(2,int(n**0.5)+1):
        #         if not n%x:
        #             return False
        #     return True
        
        # res=0
        # for i in range(n):
        #     res+=1 if isprime(i) else 0
        # return res

        if n <= 2:
            return 0

        dp=[False, False] +[True]*(n-2)

        for p in range(2, int(n**0.5)+1):
            # if dp[p]:
            for x in range(p*p,n, p):
                dp[x]=False
        return sum(dp)
86768