Minimum Time to Complete All Tasks

There are n tasks, each needing to run for a given duration within an inclusive [start, end] time window. Multiple tasks may run in the same second (shared CPU time). Find the minimum total number of seconds the CPU must be turned on to complete all tasks. Sort by end time and greedily reuse the latest seconds (sweepline / greedy, or Fenwick tree for large ranges). (LeetCode "Minimum Time to Complete All Tasks".)