The key point is that he is trying to calculate arbitrarily large fibonacci numbers, and so the addition is itself an O(n) operation. This means that even after memoization, the complexity is still O(n^2), and he uses a number of tricks to reduce that.
The key point is that he is trying to calculate arbitrarily large fibonacci numbers, and so the addition is itself an O(n) operation. This means that even after memoization, the complexity is still O(n^2), and he uses a number of tricks to reduce that.