text: class Solution(object):
      def fib(self, N):
          """
          :type N: int
          :rtype: int
          """
          
          if N < 0: return None
          
          fib = [0,1]
          
          if N < 2:
              return fib[N]
          
          N = N - 1
          while(N > 0):
              last = fib[-1]+fib[-2]
              fib.append(last)
              N-=1
          
          return fib[-1]