using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace 斐波那契数列求和

{

    class Program

    {

        static void Main(string[] args)

        {

            Console.WriteLine("第30项的值是：{0}", Fn());

            Console.WriteLine("第30项的值是：{0}", Fnl());

            Console.WriteLine("第30项的值是：{0}", Fn2());

            Console.ReadKey();

        }

        //递归算法

        static int Fn(int n)

        {

            if (n <= )

                return ;

            if (n ==  || n == )

                return ;

            return Fn(n - ) + Fn(n - );

        }

        //一般实现

        static int Fnl(int n)

        {

            if (n <= )

                return ;

            int a = ;

            int b = ;

            int c = ;

            for (int i = ; i <= n; i++)

            {

                c = checked(a + b);

                a = b;

                b = c;

            }

            return c;

        }

        //两个临时变量进行计算

        static int Fn2(int n)

        {

            if (n <= )

                return ;

            int a = ;

            int b = ;

            for (int i = ; i <= n; i++)

            {

                b = checked(a + b);

                a = b - a;

            }

            return b;

        }

    }

}