斐波那数列

剑指offer上

用递归导致多次重复的函数入出操作，时间复杂度随着n的指数方式增大的

使用for模式：

  //--斐波那数列
        static void Main(string[] args)
        {
            int n = 5;

            Program testProp = new Program();

            Console.WriteLine(testProp.GetFNBSum(n));
        }

        int GetFNBSum(int n)
        {
            if (n <= 1)
            {
                return n;
            }
            int resultSum = 0;
            int leftNum = 1;
            int rightNum = 0;
            for (int i = 2; i <= n; ++i)
            {
                resultSum = leftNum + rightNum;
                rightNum = leftNum;
                leftNum = resultSum;
            }
            return resultSum;
        }