The cons operation is called n times where n is the length of the first list when append is called. When reverse is called it calls append with n − 1 elements in the first list the first time. The first recursive call to reverse calls append with n−2 elements in the first list. The second recursive call to reverse calls append with n−3 elements in the first list. If we add up n−1+n−2+n−3+… we end up with ∑ n−1 i=1 i = ((n−1)n)/2. Multiplying this out leads to an n 2 term and the overall complexity of reverse is O(n 2 ).