Leetcode 3628. Maximum Number of Subsequences After One Inserting

Given an uppercase string s, you may insert at most one uppercase letter anywhere — determine the maximum number of subsequences equal to "LCT" that can appear in the resulting string. The core challenge is to count existing "LCT" subsequences and choose the optimal single-character insertion (letter and position) to maximize the total, with s length up to 1e5.