Perfect Diamond

using System;

namespace PerfectDiamond
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            for (int i = 1; i <= n; i++)
            {
                Console.Write(new string(' ', n - i) + '*');
                for (int j = 1; j < i; j++)
                {
                    Console.Write("-*");
                }
                Console.WriteLine();
            }

            for (int i = 1; i < n; i++)
            {
                Console.Write(new string(' ', i) + '*');
                for (int j = n - 2; j >= i; j--)
                {
                    Console.Write("-*");
                }
                Console.WriteLine();
            }
        }
    }
}

Решение със StringBuilder():

using System;
using System.Text;

namespace PerfectDiamond
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            for (int i = 0; i < n; i++)
            {
                Console.WriteLine(new string(' ', n - (i + 1)) + '*' + new StringBuilder().Insert(0, "-*", i));
            }

            for (int i = n - 2; i >= 0; i--)
            {
                Console.WriteLine(new string(' ', n - (i + 1)) + '*' + new StringBuilder().Insert(0, "-*", i));
            }
        }
    }
}