剑指Offer(七)：斐波那契数列

解法1：这道题递归很好写，但是存在很严重的效率问题。

class Solution:
    def Fibonacci(self, n):
        # write code here
        if n == 0:
            return 0
        if n == 1:
            return 1
        return self.Fibonacci(n-2)+self.Fibonacci(n-1)

解法2：循环

class Solution:
    def Fibonacci(self, n):
        # write code here
        if n <= 1:
            return n
        a = 0
        b = 1
        c = 0
        for i in range(2,n+1):
            c = a + b
            a = b
            b = c
        return c