for语句之打印三角形问题

Console.Write("请输入要打印几行：");
            int a = Convert.ToInt32(Console.ReadLine());

            for (int i = 1; i <= a; i++)
            {
                for (int j = 1; j <= i; j++)
                {
                    Console.Write("☆");
                }
                Console.WriteLine();
            }

Console.Write("请输入要打印几行：");
            int a = Convert.ToInt32(Console.ReadLine());

            for (int i = 1; i <= a; i++)
            {
                for (int j = a - i; j >= 0; j--)
                {
                    Console.Write("☆");
                }

                Console.WriteLine();
            }

 Console.Write("请输入要打印几行：");
            int a = Convert.ToInt32(Console.ReadLine());

            for (int i = 0; i < a; i++)
            {
                for (int j = 1; j <= a - (i + 1); j++)
                {
                    Console.Write("  ");
                }
                for (int k = 1; k <= i + 1; k++)
                {
                    Console.Write("☆");
                }
                Console.WriteLine();
            }

 Console.Write("请输入要打印几行：");
            int a = Convert.ToInt32(Console.ReadLine());

            for (int i = 1; i <= a; i++)
            {
                for (int k = 1; k <= a-i; k++)
                {
                    Console.Write("  ");
                }
                for (int x = 1; x <= i*2-1; x++)
                {
                    Console.Write("☆");
                }
                Console.WriteLine();
            }
            for (int j = 1; j < a; j++)
            {
                for (int k2 = 1; k2 <= j; k2++)
                {
                    Console.Write("  ");
                }
                for (int x2 = 1; x2 <= (a-j)*2-1; x2++)
                {
                    Console.Write("★");
                }
                Console.WriteLine();
            }