Find the K-Beauty of a Number
<p>The <strong>k-beauty</strong> of an integer <code>num</code> is defined as the number of <strong>substrings</strong> of <code>num</code> when it is read as a string that meet the following conditions:</p>

<ul>
	<li>It has a length of <code>k</code>.</li>
	<li>It is a divisor of <code>num</code>.</li>
</ul>

<p>Given integers <code>num</code> and <code>k</code>, return <em>the k-beauty of </em><code>num</code>.</p>

<p>Note:</p>

<ul>
	<li><strong>Leading zeros</strong> are allowed.</li>
	<li><code>0</code> is not a divisor of any value.</li>
</ul>

<p>A <strong>substring</strong> is a contiguous sequence of characters in a string.</p>

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

<pre>
<strong>Input:</strong> num = 240, k = 2
<strong>Output:</strong> 2
<strong>Explanation:</strong> The following are the substrings of num of length k:
- &quot;24&quot; from &quot;<strong><u>24</u></strong>0&quot;: 24 is a divisor of 240.
- &quot;40&quot; from &quot;2<u><strong>40</strong></u>&quot;: 40 is a divisor of 240.
Therefore, the k-beauty is 2.
</pre>

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

<pre>
<strong>Input:</strong> num = 430043, k = 2
<strong>Output:</strong> 2
<strong>Explanation:</strong> The following are the substrings of num of length k:
- &quot;43&quot; from &quot;<u><strong>43</strong></u>0043&quot;: 43 is a divisor of 430043.
- &quot;30&quot; from &quot;4<u><strong>30</strong></u>043&quot;: 30 is not a divisor of 430043.
- &quot;00&quot; from &quot;43<u><strong>00</strong></u>43&quot;: 0 is not a divisor of 430043.
- &quot;04&quot; from &quot;430<u><strong>04</strong></u>3&quot;: 4 is not a divisor of 430043.
- &quot;43&quot; from &quot;4300<u><strong>43</strong></u>&quot;: 43 is a divisor of 430043.
Therefore, the k-beauty is 2.
</pre>

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

<ul>
	<li><code>1 &lt;= num &lt;= 10<sup>9</sup></code></li>
	<li><code>1 &lt;= k &lt;= num.length</code> (taking <code>num</code> as a string)</li>
</ul>
Math; String; Sliding Window
class Solution {
public:
    int divisorSubstrings(int num, int k) {
        int ans = 0;
        string s = to_string(num);
        for (int i = 0; i < s.size() - k + 1; ++i) {
            int t = stoi(s.substr(i, k));
            ans += t && num % t == 0;
        }
        return ans;
    }
};