Count Number of Homogenous Substrings
<p>Given a string <code>s</code>, return <em>the number of <strong>homogenous</strong> substrings of </em><code>s</code><em>.</em> Since the answer may be too large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>

<p>A string is <strong>homogenous</strong> if all the characters of the string are the same.</p>

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

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

<pre>
<strong>Input:</strong> s = &quot;abbcccaa&quot;
<strong>Output:</strong> 13
<strong>Explanation:</strong> The homogenous substrings are listed as below:
&quot;a&quot;   appears 3 times.
&quot;aa&quot;  appears 1 time.
&quot;b&quot;   appears 2 times.
&quot;bb&quot;  appears 1 time.
&quot;c&quot;   appears 3 times.
&quot;cc&quot;  appears 2 times.
&quot;ccc&quot; appears 1 time.
3 + 1 + 2 + 1 + 3 + 2 + 1 = 13.</pre>

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

<pre>
<strong>Input:</strong> s = &quot;xy&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The homogenous substrings are &quot;x&quot; and &quot;y&quot;.</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;zzzzz&quot;
<strong>Output:</strong> 15
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s</code> consists of lowercase letters.</li>
</ul>
Math; String
class Solution {
public:
    const int mod = 1e9 + 7;

    int countHomogenous(string s) {
        int n = s.size();
        long ans = 0;
        for (int i = 0, j = 0; i < n; i = j) {
            j = i;
            while (j < n && s[j] == s[i]) ++j;
            int cnt = j - i;
            ans += 1ll * (1 + cnt) * cnt / 2;
            ans %= mod;
        }
        return ans;
    }
};