不使用递归

static int fun(int x1, int x2, int frequency)        {            int temp = 0;            if (frequency <= 0)                return 0;            else if (frequency == 1)                return x1;            else if (frequency == 2)                return x2;            else            {                for (int i = frequency - 2; i >= 1; i--)                {                    temp = x1 + x2;                    x1 = x2;                    x2 = temp;                }                return temp;            }        }

递归

static int fun(int i)        {            if (i <= 0)            {                return 0;            }            else if (i > 0 && i <= 2)            {                return 1;            }            else            {                return fun(i - 2) + fun(i - 1);            }        }