leetcode链接：
https://leetcode.cn/problems/fei-bo-na-qi-shu-lie-lcof/soluti...
解题思路：
递归
func fib(n int) int { if n == 0 || n == 1 { return n } a,b := 0, 1 for i := 2; i <= n; i++ { a, b = b, (a+b) % 1000000007 } return b}