Merge overlapping intervals in Java

Java interview exercise on Algorithms & recursion, difficulty 3 of 3. Solve it in the editor beside a worked example; the tests run the moment you do.

Write mergeIntervals(intervals) that merges every overlapping or touching {start, end} pair and returns the result sorted by start. Sort a copy by start, then walk it: extend the last merged interval when the next one starts at or before its end, otherwise append.

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