performance tag should be added

C#: method name should be PascalCase (Please refer to implementation of backward compatibility here )

A List is confusingly named arr, ArrayLists are passed. Either the type should be ArrayList or other Lists should be passed, including ones with a linear lookup time.

0 <= n, m <= 100 000

Missing edge case performance tests like:

List<Integer> arr = Collections.nCopies(100000, 0);
List<Integer> indexes = Collections.nCopies(100000, -1);
List<int[]> expected = Collections.nCopies(100000, new int[]{-1, -1});