Given a string `s`, return the length of the longest substring between two equal characters, excluding the two characters. If there is no such substring return `-1`.

A substring is a contiguous sequence of characters within a string.

Example 1:

    Input: s = "aa"
    Output: 0
    Explanation: The optimal substring here is an empty substring between the two 'a's.

Example 2:

    Input: s = "abca"
    Output: 2
    Explanation: The optimal substring here is "bc".

Example 3:

    Input: s = "cbzxy"
    Output: -1
    Explanation: There are no characters that appear twice in s.

Constraints:

- `1 <= s.length <= 300`
- `s` contains only lowercase English letters.