Unique Length-3 Palindromic Subsequences
Given a string s, return the number of unique palindromes of length three that are a subsequence of s.
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.
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 <= 10^5
- s consists of only lowercase English letters.
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