<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:
    bool isPrime[10000000];
    int countPrimes(int n) {
        for(int i=0;i<=n;i++) isPrime[i] = 1;
        isPrime[0] = isPrime[1] = 0;
        for(int i=2;i*i<=n;i++)
        {
            if(isPrime[i])
            {
                for(int j = i*i;j<=n;j+=i)
                    isPrime[j] = false;
            }
        }
        int cnt = 0;
        for(int i=0;i<n;i++)
            cnt+=isPrime[i];
        return cnt;
        
    }
};
18900