雷：面试时被问到fibonacci

1 1 2 3 5 8...

递归：

        static int FibonacciRecursive(int n)
        {        
            if (n == 1 || n == 2)
                return 1;
            else if (n >= 2)
            {
                return FibonacciRecursive(n - 1) + FibonacciRecursive(n - 2);
            }
            else if (n <= 0)
            {
                throw new ArgumentNullException(string.Format("{0} is illegal", n));              
            }
            return 0;
        }

非递归：

        static int Fibonacci(int n)
        {
            int lastNumber=1;
            int secondNumber=1;
            int currentNumber = 1;
            if (n > 0)
            {
                for (int i = 3; i <= n; i++)
                {                   
                    currentNumber = lastNumber+secondNumber;
                    secondNumber = lastNumber;
                    lastNumber = currentNumber;
                }
            }
            else
            {
                throw new ArgumentNullException(string.Format ("{0} is not a legal number",n));
            }
            return currentNumber;
        }