Usually a fine approach would be to construct a hash table as we walk the array, and if the element is not seen put it in the hash table, otherwise "remove" it by setting it to the empty string. If the hash table implementation is good, we can pretty much find out whether an entry has been seen before in O(1) time, making the whole process O(n).
That's what the author called "alt" and I don't understand why the author objects to it. The memory needed is usually proportional to the number of distinct elements in the original array. You don't generally have to copy the original values (usually you just copy a pointer).
That's what the author called "alt" and I don't understand why the author objects to it. The memory needed is usually proportional to the number of distinct elements in the original array. You don't generally have to copy the original values (usually you just copy a pointer).