The author seems to start promising with the 2 given solutions on the first provided example and in addition talking about time complexity. Then the author immediately disqualifies himself as a reliable source on the matter of “JavaScript Data Structure[s]” with the worst possible implementation of `findUniqueNos` provided as normal function (not as callback of an array method) with an inner mapping process and an additionally nested `filter` … really? `map` and `filter`? Why not at least `forEach` (correct method) and `find` (improved time complexity) which still is far from an ideal solution? Better was already `forEach` and a map/object based lookup (improved time complexity again) instead of the `uniqueNumbers` array. Equally good as the last suggested approach is an implementation as a `reduce` method’s callback function which would use a lookup based approach too.