[beeffective]

In my personal experience, if this question were posed to me by a project team, I would ask questions about usage and purpose (granted it would be more business-like such as given an array of 10 million transcript credits, does employee X exist for the Code of Conduct training). If knowing whether a particular number (or item) existed in a large array, maybe we then create a boolean and check during inserts/updates/deletes. Who knows, lets ask more questions and understand each other. Not every developer works on cutting edge Google Search algorithms in lonely cubicle all day by his lonesome.

As a developer, understanding why and coming to an efficient solution (in terms of development hours and customer satisfaction) is about problem decomposition and problem solving skills, not being an algorithm expert.