public static int Foo(int i)
        {
            if (i <= 0)
                return 0;
            else if (i <= 2)
                return 1;
            else
            {
                return Foo(i - 1) + Foo(i - 2);
            }
        }请问这个算法要怎么理解,它的执行过程是怎么样的呢?为什么Foo(6)等于8呢?