Substrings That Begin and End With the Same Letter
<p>You are given a <strong>0-indexed</strong> string <code>s</code> consisting of only lowercase English letters. Return <em>the number of <strong>substrings</strong> in </em><code>s</code> <em>that begin and end with the <strong>same</strong> character.</em></p>

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

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

<pre>
<strong>Input:</strong> s = &quot;abcba&quot;
<strong>Output:</strong> 7
<strong>Explanation:</strong>
The substrings of length 1 that start and end with the same letter are: &quot;a&quot;, &quot;b&quot;, &quot;c&quot;, &quot;b&quot;, and &quot;a&quot;.
The substring of length 3 that starts and ends with the same letter is: &quot;bcb&quot;.
The substring of length 5 that starts and ends with the same letter is: &quot;abcba&quot;.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abacad&quot;
<strong>Output:</strong> 9
<strong>Explanation:</strong>
The substrings of length 1 that start and end with the same letter are: &quot;a&quot;, &quot;b&quot;, &quot;a&quot;, &quot;c&quot;, &quot;a&quot;, and &quot;d&quot;.
The substrings of length 3 that start and end with the same letter are: &quot;aba&quot; and &quot;aca&quot;.
The substring of length 5 that starts and ends with the same letter is: &quot;abaca&quot;.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;a&quot;
<strong>Output:</strong> 1
<strong>Explanation:</strong>
The substring of length 1 that starts and ends with the same letter is: &quot;a&quot;.
</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 only of lowercase English letters.</li>
</ul>
Hash Table; Math; String; Counting; Prefix Sum
class Solution {
    public long numberOfSubstrings(String s) {
        int[] cnt = new int[26];
        long ans = 0;
        for (char c : s.toCharArray()) {
            ans += ++cnt[c - 'a'];
        }
        return ans;
    }
}
