https://www.nowcoder.net/practice/c6c7742f5ba7442aada113136ddea0c3?tpId=13&tqId=11160&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking

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。

n<=39

class Solution {public:    int Fibonacci(int n) {        if (n < 1) return 0;        if (n <= 2) return 1;        int fib2 = 1;        int fib1 = 1;        for (int i=3; i<=n; i++) {            int tmp = fib2 + fib1;            fib2 = fib1;            fib1 = tmp;        }        return fib1;    }};