509. Fibonacci Number

class Solution:
    def fib(self, N: int) -> int:
        x1 = 0
        x2 = 1

        if N == 0:
            return x1
        elif N == 1:
            return x2
        else:
            res = x1

        while N > 1:
            res = x1 + x2
            x1 = x2
            x2 = res
            N -= 1

        return res

#     def fib(self, N: int) -> int:
#         if N == 0:
#             return 0
#         elif N == 1:
#             return 1
#         else:
#             return self.fib(N - 1) + self.fib(N - 2)

#     def fib(self, N: int) -> int:
#         cache = {}
#         
#         def cache_fib(N):
#             if N in cache:
#                 return cache[N]

#             if N == 0:
#                 res = 0
#             elif N == 1:
#                 res = 1
#             else:
#                 res = cache_fib(N - 1) + cache_fib(N - 2)

#            # put result in cache for later reference.
#            cache[N] = res
#            return res

#        return cache_fib(N)