1930. Unique Length-3 Palindromic Subsequences

https://leetcode.com/problems/unique-length-3-palindromic-subsequences

Description

Given a string s, return the number of unique palindromes of length three that are a subsequence of s.

Note that even if there are multiple ways to obtain the same subsequence, it is still only counted once.

A palindrome is a string that reads the same forwards and backwards.

A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

• For example, "ace" is a subsequence of "abcde".

Example 1:

**Input:** s = "aabca"
**Output:** 3
**Explanation:** The 3 palindromic subsequences of length 3 are:
- "aba" (subsequence of "aabca")
- "aaa" (subsequence of "aabca")
- "aca" (subsequence of "aabca")

Example 2:

**Input:** s = "adc"
**Output:** 0
**Explanation:** There are no palindromic subsequences of length 3 in "adc".

Example 3:

**Input:** s = "bbcbaba"
**Output:** 4
**Explanation:** The 4 palindromic subsequences of length 3 are:
- "bbb" (subsequence of "bbcbaba")
- "bcb" (subsequence of "bbcbaba")
- "bab" (subsequence of "bbcbaba")
- "aba" (subsequence of "bbcbaba")

Constraints:

• 3 <= s.length <= 105

• s consists of only lowercase English letters.

ac