剑指 Offer 10- I. 斐波那契数列

剑指 Offer 10- I. 斐波那契数列

思路：

开三个变量存三个数，下一次迭代first的时候是上一轮的mid 和 end

代码：

class Solution {
public:
    int fib(int n) {
        int MOD = 1e9+7;
        if (n < 2) {
            return n;
        }
        int first = 0, mid = 0, end = 1;
        for (int i = 2; i <= n; ++i) {
            first = mid; 
            mid = end; 
            end = (first + mid)%MOD;
        }
        return end;
    }
};