Unique Length-3 Palindromic Subsequences In Strings
We can use an efficient algorithm that leverages prefix and suffix character tracking to count all valid palindromic subsequences in O(n) time complexity.
1930. Unique Length-3 Palindromic Subsequences Difficulty: Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum 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, "...