LeetCode 516 Longest Palindromic Subsequence - Problem Statement

Given a string $s$, find the length of the longest subsequence that reads the same forward and backward. Example: for $s =$ "bbbab", the answer is $4$ (the subsequence "bbbb").

This is your first interval DP problem. The important point: any palindromic subsequence on $s[i..j]$ depends on whether $s[i] = s[j]$. If they match, both characters can be part of the palindrome. If not, at least you must be skipped. Before reading on, think: what's the simplest base case? What happens when $i = j$?