<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 countPrimes(int n) {
        if(n<=2)return 0;
        long long unsigned int a[n];
        for(long int i=0;i<n;i++)a[i]=1;
        a[0]=0;
        a[1]=0;
        int c=1;
        for(long int i=3;i<n;i+=2){
            if (a[i]==1){
                c++;
                long long int j=i*i;
                while(j<n){
                    a[j]=0;
                    j=j+i;
                }}
        }
        return c;
    }
};
51660