(4) There are N stairs and Alice can climb one or two stairs at a time. How many different ways can Alice climb the stairs such that she starts at the bottom and ends at the top? The algorithm does not need to explicitly list the ways. First devise a top-down (recursive) approach to solve this problem. What is its runtime complexity if intermediate results are not stored and not reused? How does memoizing intermediate results change the complex- ity? Next devise a bottom-up DP approach to solve this problem. What is its runtime complexity