Leetcode 3639. Minimum Time to Activate String

Given a permutation order that replaces characters with '*' over time, find the minimum t such that the number of substrings containing at least one '*' is ≥ k. Key idea: the condition is monotonic in t, so binary-search t and check in O(n) by computing valid substrings as total substrings minus the sum of substrings inside contiguous non-'*' segments.