<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:
    void seive(int n,vector<bool> &ans)
    {
        for(int i=2;i*i<n;i++)
        {
            if(ans[i]==1)
           { 
            for(int j=i*i;j<=n;j+=i)
            {
                ans[j]=0;
            }
            }
        }
    }
    int countPrimes(int n) {
        vector<bool> ans(1e7+1,1);
        seive(n+10,ans);
        int res=0;
        for(int i=2;i<n;i++)
        {
            if(ans[i]==1) res++;
        }
        return res;
    }
};
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