Find the Count of Numbers Which Are Not Special
<p>You are given 2 <strong>positive</strong> integers <code>l</code> and <code>r</code>. For any number <code>x</code>, all positive divisors of <code>x</code> <em>except</em> <code>x</code> are called the <strong>proper divisors</strong> of <code>x</code>.</p>

<p>A number is called <strong>special</strong> if it has exactly 2 <strong>proper divisors</strong>. For example:</p>

<ul>
	<li>The number 4 is <em>special</em> because it has proper divisors 1 and 2.</li>
	<li>The number 6 is <em>not special</em> because it has proper divisors 1, 2, and 3.</li>
</ul>

<p>Return the count of numbers in the range <code>[l, r]</code> that are <strong>not</strong> <strong>special</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">l = 5, r = 7</span></p>

<p><strong>Output:</strong> <span class="example-io">3</span></p>

<p><strong>Explanation:</strong></p>

<p>There are no special numbers in the range <code>[5, 7]</code>.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">l = 4, r = 16</span></p>

<p><strong>Output:</strong> <span class="example-io">11</span></p>

<p><strong>Explanation:</strong></p>

<p>The special numbers in the range <code>[4, 16]</code> are 4 and 9.</p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= l &lt;= r &lt;= 10<sup>9</sup></code></li>
</ul>
Array; Math; Number Theory
const int m = 31623;
bool primes[m + 1];

auto init = [] {
    memset(primes, true, sizeof(primes));
    primes[0] = primes[1] = false;
    for (int i = 2; i <= m; ++i) {
        if (primes[i]) {
            for (int j = i * 2; j <= m; j += i) {
                primes[j] = false;
            }
        }
    }
    return 0;
}();

class Solution {
public:
    int nonSpecialCount(int l, int r) {
        int lo = ceil(sqrt(l));
        int hi = floor(sqrt(r));
        int cnt = 0;
        for (int i = lo; i <= hi; ++i) {
            if (primes[i]) {
                ++cnt;
            }
        }
        return r - l + 1 - cnt;
    }
};