The problem asks to find k cards (integers) out of n, in time O(k) and O(1) additional space — this means additional to the k required to store the output. So w

The problem asks to find k cards (integers) out of n, in time O(k) and O(1) additional space — this means additional to the k required to store the output. So while holding n items wouldn't fit in memory, it is implicit that k would. But we still want to avoid Ω(k log k) time, or more than a constant amount of additional space.