上楼梯算法

        /// <summary>
        /// 上楼梯算法（迭代）
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        private static long fib3(int n)
        {
            long x = 0, y = 0, z = 1;
            long w, k;

            for (int j = 0; j < n; j++)
            {
                w = z;
                k = y;
                z = x + y + z;
                y = w;
                x = k;
            }
            return z;
        }

        /// <summary>
        /// 上楼梯算法（递归）
        /// </summary>
        /// <param name="n"></param>
        /// <returns></returns>
        private static long dfib1(int n)
        {
            if (n < 1)
                return 0; 
            if (n == 1)
                return 1;
            if (n == 2)
                return 2;
            if (n == 3)
                return 4;

            return dfib1(n - 1) + dfib1(n - 2) + dfib1(n - 3);
        }