斐波那契数列规则：第1、2项为1，后续各项为前两项之和，函数fibo的实现代码如下：

int fibo(int n) {
    if (n <= 0)
        return 0;
    if (n == 1 || n == 2)
        return 1;
    int a = 1, b = 1, next;
    for (int i = 3; i <= n; i++) {
        next = a + b;
        a = b;
        b = next;
    }
    return next;
}