Efficiency of each of the following calls for each type of structure
O(1), O(N), O(N^2), ...
                        add(el)       find(el)        remove(el)
unsorted array:          O(1)           O(n)            O(n)
sorted array:            O(n)         O(log n)          O(n)
unsorted Linked List:    O(1)           O(n)            O(n)
sorted Linked List:     O(log n)        O(n)            O(n)
hashset:                 O(1)           O(1)            O(1)