Solution: Count Palindromic Subsequences

Statement

Given a string s made up only of digit characters, count how many subsequences of s form a palindrome of length $5$.

Return the total number of such palindromic subsequences.

Note:

• A string is palindromic if it reads the same forward and backward.

• A subsequence is obtained by deleting zero or more characters from s without changing the relative order of the remaining characters.

• A subsequence is defined by the indices chosen from s, so identical resulting strings formed from different index selections are counted multiple times.

Constraints:

• $1 \leq$ ...