Note that the previous answer does not keep the original order of the numbers (which might be required). To keep the order, use auxiliary hash set (unordered set) to check if the coming number is a duplicate or not. And it is O(n) (assuming the frequency of hash collisions is low). Or ordered set for guaranteed O(n * log n) solution. That requires auxiliary structure with O(n) memory, however the sort solution uses auxiliary memory as well (virtually every O(n*logn) runtime sort needs at least O(logn) space, might it be space on the stack).