<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>

bool arr[10000000] = {0};

class Solution {
public:
    int countPrimes(int n) {
        if (n <= 1)
            return 0;
        for (bool& b : arr)
            b = true;
        int ans = -2, i = 2;
        while (i < pow(n,0.5f)) {
            for (int j = i * i; j < n; j += i)
                arr[j] = false;
            while (!arr[++i])
                ;
        }
        for (int i=0;i<n;i++)
            if (arr[i]) ans++;
        return ans;
    }
};
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