for语句之打印三角形问题

1.左下角直角三角形

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

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

2.左上角直角三角形

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

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

                Console.WriteLine();
            }

3.右下角直角三角形

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

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

4.菱形

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

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