<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
        def setprimes(n):
            global sieve
            sieve=[1]*(n+1)
            sieve[0]=sieve[1]=0
            for i in range(2,1+int(sqrt(n))):
                if sieve[i]:
                    for j in range(i*i,n+1,i):
                        sieve[j]=0
        setprimes(n)
        return sum(sieve)-sieve[-1]
        # pref=sieve.copy()
        # for i in range(1,len(pref)):
        #     pref[i]+=pref[i-1]
        # return pref[n]-sieve[n]
        
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