斐波那契

# -*- coding:utf-8 -*-
class Solution:
    def Fibonacci(self, n):
        # write code her
        if n <= 1:
            return n
        if n == 2:
            return 1
        a = 1
        b = 1
        sum = 0
        for i in range(n-2):
            sum = a + b
            a = b
            b = sum
        return sum