SpiralTraverseMatrix

using System;
using System.Collections.Generic;

namespace Spiral
{
    class Program
    {
        static bool IsFree(int[,] b, int i, int j)
        {
            int n = b.GetLength(0);
            int m = b.GetLength(1);
            if (i < 0 || i >= n || j < 0 || j >= m)
            {
                return false;
            }
            else if (b[i, j] != 0)
            {
                return false;
            }
            return true;
        }
        static List<int> SpiralTraverse(int[,] a)
        {
            List<int> nums = new List<int>();
            int[,] b = new int[a.GetLength(0), a.GetLength(1)];

            int i = 0;
            int j = 0;
            int rounds = Math.Min(a.GetLength(0), a.GetLength(1)) / 2;
            for (int k = 0; k <= rounds; k++)
            {
                while(IsFree(b, i, j))
                {
                    nums.Add(a[i, j]);
                    b[i, j] = 1;
                    j++;            // right
                }
                i++;
                j--;
                while (IsFree(b, i, j))
                {
                    nums.Add(a[i, j]);
                    b[i, j] = 1;
                    i++;          // down
                }
                i--;
                j--;
                while (IsFree(b, i, j))
                {
                    nums.Add(a[i, j]);
                    b[i, j] = 1;
                    j--;           // left
                }
                i--;
                j++;
                while (IsFree(b, i, j))
                {
                    nums.Add(a[i, j]);
                    b[i, j] = 1;
                    i--;         // up
                }
                i++;
                j++;
            }
            return nums;
        }
        static void Main(string[] args)
        {
            int[,] matrix =   { { 1, 2, 3, 4, 5 },
                                { 6, 7, 8, 9, 10 },
                                { 11, 12, 13, 14, 15 },
                                { 16, 17, 18, 19, 20 },
                                { 21, 22, 23, 24, 25 }};

            List<int> spiral = SpiralTraverse(matrix);
            foreach (var item in spiral)
            {
                Console.Write($"{item} ");
            }
            Console.WriteLine();
        }
    }
}