<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 {
public:
    int N=5*1e6+5;
    int f(int n){
        vector<bool>is_prime(N,true);
        is_prime[0]=is_prime[1]=false;
        for(int i=2;i*i<n;i++){
            if(is_prime[i]){
                for(int j=i*i;j<n;j+=i){
                      is_prime[j]=false;
                }
            }
        }
        int num=0;
        for(int i=2;i<n;i++){
            if(is_prime[i])num++;
        }
        return num;
    }
    int countPrimes(int n) {
        return f(n);
    }
};
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