Leetcode 2484. Count Palindromic Subsequences

Count the number of length-5 palindromic subsequences in a digit string (they must have the form a b c b a) and return the result modulo 1e9+7. The algorithmic challenge is to efficiently count combinations of matching outer and inner digit pairs across positions (e.g., via aggregated prefix/suffix counts or DP) for strings up to ~10^4 in length.