Leetcode 2131. Longest Palindrome by Concatenating Two Letter Words

Given a list of two-letter words, build the longest palindrome by selecting words so that reverse pairs (e.g., "ab" and "ba") contribute 4 characters each and at most one identical-letter word (e.g., "aa") can sit in the center for +2. The core challenge is counting and matching reverse pairs and optionally placing a single symmetric word as the center to maximize length.