[转]C#算法 Fibonacci费波纳数列

1.   1,1,2,3,5,8,13,21,34....求其第30位是多少？

        /// <summary>
        /// 1,1,2,3,5,8,13,21,34....求其第30位是多少？
        /// 递归算法
        /// </summary>
        /// <param name="i"></param>
        /// <returns></returns>
        public static int Fibo(int i)
        {
            if (i == 0)
                return 1;
            else if (i == 1)
                return 1;
            else
                return Fibo(i - 1) + Fibo(i - 2);
        }

            //递归算法
              Console.WriteLine(Fibo(6));
            //不用递归
              int[] num = new int[30];
            num[0] = 1;
            num[1] = 1;
            int first = num[0];
            int second = num[1];
            for (int i = 2; i < num.Length; i++)
            {
                num[i] = first + second;
                first = second;
                second = num[i];
            }
            Console.WriteLine(num[6]);

2. 

        /// <summary>
        /// 1+2+3+4+5+...+100
        /// </summary>
        /// <param name="i"></param>
        /// <returns></returns>
        public static int Fibo2(int i)
        {
            if (i == 1)
                return 1;
            else
                return Fibo2(i - 1) + i;
        }

        /// <summary>
        /// 1-2+3-4+5-6+7-8+9
        /// </summary>
        /// <param name="i"></param>
        /// <returns></returns>
        public static int Fibo3(int i)
        {
            if (i == 1) return 1;
            if (i == 2) return 2;
            else
                return Fibo3(i - 2) + i;
        }

Console.WriteLine(Fibo2(100));
Console.WriteLine(Fibo3(9) - Fibo3(8));