1、斐波那切数列（循环法O(n)）
大家都知道斐波那契数列，现在要求输入一个整数n，请你输出斐波那契数列的第n项（从0开始，第0项为0）。
n<=39
2、代码详解
# -*- coding:utf-8 -*-
class Solution:
def Fibonacci(self, n):
fibN_Minus_One = 1
fibN_Minus_Two = 0
if n == 0:
return fibN_Minus_Two
elif n == 1:
return fibN_Minus_One
for i in range(2, n+1):
# f(n)=f(n-1)+f(n-2)
fibN = fibN_Minus_One + fibN_Minus_Two
# f(n-2) = f(n-1)
fibN_Minus_Two = fibN_Minus_One
# f(n-1) = f(n)
fibN_Minus_One = fibN
return fibN
