<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 checkPrime(int x)
    {
        if (x <= 1)
            return 0;
        else
            for (int i = 2; i <= sqrt(x); i++)
                if (x % i == 0)
                    return 0;
        return 1;
    }
    int countPrimes(int n) {
        bool *isprime = new bool[1e7];
        isprime[0] = isprime[1] = 0;
        for (int i = 2; i < n; i++)
            isprime[i] = 1;
        for (int i = 2; i <= sqrt(n); i++)
            if (checkPrime(i) == 1)
            {
                for (int j = i * i; j < n; j = j + i)
                    isprime[j] = 0;
            }
        int count = 0;
        for (int i = 2; i < n; i++)
            if (isprime[i])
                count++;
        return count;
    }
};
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