<p>Given a string <code>s</code>, return <em>the length of the longest substring between two equal characters, excluding the two characters.</em> If there is no such substring return <code>-1</code>.</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;aa&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> The optimal substring here is an empty substring between the two <code>&#39;a&#39;s</code>.</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abca&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The optimal substring here is &quot;bc&quot;.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;cbzxy&quot;
<strong>Output:</strong> -1
<strong>Explanation:</strong> There are no characters that appear twice in s.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 300</code></li>
	<li><code>s</code> contains only lowercase English letters.</li>
</ul>

class Solution {
public:
    int maxLengthBetweenEqualCharacters(string s) {
        vector<vector<int>> vec(26);
        int n = s.length();
        for(int i=0; i<n; i++) {
            int idx = s[i] - 'a';
            vec[idx].push_back(i);
        }
        
        int maxx = -1;
        
        for(int i=0; i<26; i++) {
            if(vec[i].size() < 2) continue;
            int st = vec[i][0];
            int en = vec[i][vec[i].size()-1];
            maxx = max(maxx, en-st-1);
        }
        return maxx;
    }
};
8100