<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 {
    int index(char ch) { return ch - 'a';}
public:
    int maxLengthBetweenEqualCharacters(string s)
    {
        vector<int> start_pos(26, -1);
        int longest_substring = -1;

        for (int i = 0, size = s.length(); i != size; ++i) {
            int& pos = start_pos[ index( s[i] ) ];
            if ( pos == -1 ) pos = i;
            else {
                longest_substring = max( longest_substring, i - pos - 1 );
            }
        }
        return longest_substring;
        
    }
};
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