Solution: Unique Length-3 Palindromic Subsequences

Statement

Given a string s, return the count of unique palindromic subsequences of length $3$ that exist within s.

Even if a particular subsequence can be formed in multiple ways, it is counted only once.

A palindrome is a string that reads the same forwards and backwards. A subsequence is derived from the original string by deleting zero or more characters without altering the relative order of the remaining characters.

Constraints:

• $3 \leq$ s.length $$ ...