I recommend adding more unit testing.

Re-raising the issue below.
Random tests occasionally produce duplicates, and previous approach pass the test Passed: 1 Failed: 5 Exit Code: 1

This kata has performance requirements but the input range is not specified.

(Also, performance tag should be needed)

This is a C++ kata, not C, so the kata design should follow C++ standards, e.g std::vector<int> should be used in place of int[], int as array content and length.