Leetcode 1541. Minimum Insertions to Balance a Parentheses String

Given a string of '(' and ')', return the minimum number of insertions needed to make it balanced under the rule that each '(' must be closed by two consecutive ')' (i.e., treat '))' as one closing), with insertions allowed anywhere. This is a linear-time greedy/stack-style balance problem (n up to 1e5).