求和：1-2+3-4+……+m

//1. 递归求和 GetValue2(int i)
//2. 总结规律求和 GetValue(int i)
//3. 判断奇偶求和

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SumDemo2
{
    class Program
    {
        static int GetValue2(int i)
        {
            int flag = 0;

            if (i == 1)
                return 1;
            else if (i == 2)
                return -1;

            if (i % 2 == 0)
            {
                flag = 0 - GetValue2(i - 1);
            }
            else
            {
                flag = i / 2 + 1;
            }

            return flag;
        }

        static int GetValue(int i)
        {
            int flag = 0;

            if (i == 1)
                return 1;
            else if (i == 2)
                return -1;

            if (i % 2 != 0) 
            {
                flag = i / 2 + 1;
            }
            else
            {
                flag = 0 - i / 2 ;
            }

            return flag;
        }

        static void Main(string[] args)
        {
            for (int i = 1; i < 10; i++)
            {
                Console.Write(GetValue(i));
                Console.Write("  ");
            }

            Console.WriteLine("
-----------------------------------------");

            for (int i = 1; i < 10; i++)
            {
                Console.Write(GetValue2(i));
                Console.Write("  ");
            }

            Console.WriteLine("
-----------------------------------------");

            for (int i = 1; i < 10; i++)
            {
                int sum = 0;
                for (int j = 1; j <= i; j++)
                {
                    if (j == 1)
                    {
                        sum += j;
                    }
                    else if (j % 2 == 0)
                    {
                        sum -= j;
                    }
                    else if (j % 2 == 1)
                    {
                        sum += j;
                    }
                }

                Console.Write(sum);
                Console.Write("  ");
            }

            Console.WriteLine("
-----------------------------------------");

            Console.ReadLine();
        }
    }
}