Written as an exercise for coding challenge.

• List-based merge: O(n*log n) complexity, O(n) memory
• Tree-based merge: O(n) complexity, O(n) memory
• Parallel tree-based merge: O(n) complexity, O(n) memory

Caveat: some consider big-O notation deceiving.

This is a standard Maven project, use mvn test.

There's also a profile for code coverage; run mvn test -Pcoverage. Results will be in target/site/jacoco/index.html.