# 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

```java
```
