<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 {
    #define in long long
public:
    in countPrimes(in n) {
        vector<bool> v(n+1,1);
        v[0]=v[1]=1;
        vector<in> ans;
        //n=sqrt(n);
        for(in i=2;i<n;i++){
        if(v[i]){
            ans.push_back(i);
            for(in j=i*i;j<n;j+=i) v[j]=0;
        }
      }
      in cnt=0;
      for(in i=2;i<n;i++){if(v[i])cnt++;}
      return cnt;   
    }
};
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