Rotate a matrix NxN 90 degrees

90graus.cs

using System;
using static System.Console;

namespace _90graus
{
    class Program
    {
        static void Main(string[] args)
        {
            var length = 3;
            var matrix = new string[length, length];

            for (var x = 0; x <= matrix.GetUpperBound(0); x++)
            {
                for (var y = 0; y <= matrix.GetUpperBound(0); y++)
                {
                    matrix[x, y] = (x + y).ToString();
                }
            }

            WriteLine("Original");
            PrintMatrix(matrix);
            WriteLine("");

            var up = matrix.GetUpperBound(0);
            for(var y = up; y > 0; y--)
            {
                for (var x = up -y; x < y; x++)
                {
                    var next = CalculateIndex(x, y, up);

                    var temp = matrix[next.x, next.y];
                    matrix[next.x, next.y] = matrix[x, y];

                    next = CalculateIndex(next.x, next.y, up);

                    var item = matrix[next.x, next.y];
                    matrix[next.x, next.y] = temp;

                    next = CalculateIndex(next.x, next.y, up);
                    temp = matrix[next.x, next.y];
                    matrix[next.x, next.y] = item;

                    next = CalculateIndex(next.x, next.y, up);
                    item = matrix[next.x, next.y];
                    matrix[next.x, next.y] = temp;
                }
            }

            WriteLine("Rotate 90");
            PrintMatrix(matrix);
        }

        static (int x, int y) CalculateIndex(int x, int y, int up)
        {
            return (y, up - x);
        }


        static void PrintMatrix(string[,] matrix)
        {
            for (var x = 0; x <= matrix.GetUpperBound(0); x++)
            {
                for (var y = 0; y <= matrix.GetUpperBound(0); y++)
                {
                    Write(matrix[x, y].PadRight(3));
                }

                WriteLine("");
            }
        }
    }
}