<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>

    int prime[5000000];
    bool done = false;
    void sieve(int N)
    {
        fill(prime, prime + 5000000, 1);
        for(long long i=2; i*i<N; i++)
        {
            if(prime[i])
            {
                for(long long j=i*i; j<N; j+=i)
                {
                    prime[j] = 0;
                }
            }
        }
    }
class Solution {
public:
    int countPrimes(int n) {
        if(!done){
            sieve(5000000);
            done = true;
        }

        int count = 0;
        for(int i=2; i<n; i++)
        {
            if(prime[i]) count++;
        }
        return count;
    }
};
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