Matrix Shuffle

/* You are given an input string which you should fill in a square spiral matrix with a given size. After filling up the matrix you should move through it and extract from it all the letters in a chessboard pattern.
 * Your next task is to check if the newly formed sentence, read in lowercase and with all non-letter characters removed, is a palindrome (reading it from left to right is the same as reading it from right to left).
 * The first input line will hold a number – the size of the matrix. The second line will hold the text.
 * The output should be an HTML <div> tag that shows the newly found sentence, colored by changing its background to #E0000F (see the examples below) if the sentence is not a palindrome or #4FE000 if the sentence is a palindrome. */

namespace Matrix_Shuffle
{
    using System;
    using System.Linq;
    using System.Text;

    class MatrixShuffle
    {
        static void Main(string[] args)
        {
            int size = int.Parse(Console.ReadLine());
            string text = Console.ReadLine();

            char[,] matrix = new char[size, size];
            FillTheMatrix(matrix, text);
            string extracted = ExtractLettersFromMatrixInChessboardPattern(matrix);

            Console.WriteLine(
                CheckExtractedSentenceIsPalindrome(extracted)
                    ? "<div style='background-color:#4FE000'>{0}</div>"
                    : "<div style='background-color:#E0000F'>{0}</div>",
                extracted);
        }

        private static void FillTheMatrix(char[,] matrix, string text)
        {
            string direction = "right";
            int size = matrix.GetLength(0);
            int row = 0;
            int col = 0;
            for (int i = 0; i < matrix.Length && i < text.Length; i++)
            {
                matrix[row, col] = text[i];
                if (direction == "right" && (col == size - 1 || matrix[row, col + 1] != '\0'))
                {
                    direction = "down";
                }

                if (direction == "down")
                {
                    if (row == size - 1 || matrix[row + 1, col] != '\0')
                    {
                        direction = "left";
                    }
                    else
                    {
                        row++;
                    }
                }

                if (direction == "left")
                {
                    if (col == 0 || matrix[row, col - 1] != '\0')
                    {
                        direction = "up";
                    }
                    else
                    {
                        col--;
                    }
                }

                if (direction == "up")
                {
                    if (row == 0 || matrix[row - 1, col] != '\0')
                    {
                        direction = "right";
                    }
                    else
                    {
                        row--;
                    }
                }

                if (direction == "right")
                {
                    col++;
                }
            }
        }

        private static string ExtractLettersFromMatrixInChessboardPattern(char[,] matrix)
        {
            StringBuilder firstPart = new StringBuilder();
            StringBuilder secondPart = new StringBuilder();

            int size = matrix.GetLength(0);
            for (int row = 0; row < size; row++)
            {
                for (int col = 0; col < size; col++)
                {
                    if ((row % 2 == 0 && col % 2 == 0) || (row % 2 != 0 && col % 2 != 0))
                    {
                        firstPart.Append(matrix[row, col]);
                    }
                    else
                    {
                        secondPart.Append(matrix[row, col]);
                    }
                }
            }

            return (firstPart.Append(secondPart).ToString());
        }

        private static bool CheckExtractedSentenceIsPalindrome(string sentence)
        {
            StringBuilder sb = new StringBuilder();
            foreach (char ch in sentence.Where(ch => char.IsLetter(ch)))
            {
                sb.Append(char.ToLower(ch));
            }

            string leftToRight = sb.ToString();
            string reversed = ReverseString(leftToRight);

            return reversed == leftToRight;
        }

        private static string ReverseString(string text)
        {
            char[] letters = text.ToCharArray();
            Array.Reverse(letters);

            return new string(letters);
        }
    }
}