static void Main(string[] args)
        {
            //九九乘法
            Console.WriteLine("九九乘法口诀");
            for (int i = 1; i <= 9; i++)
            {
                for (int j = 1; j <= i; j++)
                {
                    Console.Write(string.Format("{0}*{1}={2}\t", i, j, i * j));
                }
                Console.WriteLine();
            }
            //输入矩形
            Console.WriteLine("矩形");
            for (int i = 0; i < 3; i++)
            {
                //除了第一行和最后一行其余中间要空

                for (int j = 0; j < 5; j++)
                {
                    if (i > 0 && i != 2)
                    {
                        if (j > 0 && j != 4)
                        {
                            Console.Write(" ");
                        }
                        else
                        {
                            Console.Write("*");
                        }
                    }
                    else
                    {
                        Console.Write("*");
                    }
                }
                Console.WriteLine();
            }


            //画一个等边三角形
            Console.WriteLine("等边三角形");
            int width = 5;

            for (int i = 1; i <= 5; i++)
            {
                for (int k = 1; k <= 5 - i; k++)
                {
                    Console.Write(" ");
                }
                for (int j = 1; j <= i; j++)
                {
                    Console.Write(" ");
                    Console.Write("*");

                }

                Console.WriteLine();
            }

            //直角三角形
            Console.WriteLine("直角三角形");
            for (int i = 0; i < 5; i++)
            {
                for (int j = 0; j < i; j++)
                {
                    Console.Write("*");
                }
                Console.WriteLine();
            }

            Console.ReadKey();
        }

 

 Console.WriteLine("1-2+3-4...100结果");
            int sumO = 0;
            int sumJ = 0;
            for (int i = 0; i < 100; i++)
            {
                //索引0正好为奇数，奇数正，偶数负
                if(i % 2 == 0)
                {
                    sumO += i;
                }
                else
                {
                    sumJ += -i;
                }
            }
            Console.WriteLine(string.Format("{0}",sumJ+sumO));