Give an example where choosing the right data structure changes the Big-O.

Checking membership: `ArrayList.contains` scans the list, so doing it inside a loop is O(n^2); switching to a `HashSet`, whose contains is O(1) average, makes the same task O(n).

Likewise, searching an unsorted array is O(n) but binary search on a SORTED array is O(log n). The algorithm's complexity often comes down to which structure you store the data in, not how tightly you write the loop.