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Solution: Unique Length-3 Palindromic Subsequences
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Problem Statement

Given a string s, return the total number of unique palindromes of length 3 which are subsequences of s. Even if multiple ways exist to obtain the same subsequence, it is still only counted once.

A palindrome string is a sequence of characters that reads the same backward as forward.

A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements.

Examples

Example 1:

  • Input: s = "abcba"
  • Expected Output: 3

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