Leetcode 54 - spiral matrix - recursive solution

Leetcode54_SpiralMatrix_Recursive.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Sprila_matrix_recursive
{
    /// <summary>
    /// Leetcode 54: Spiral matrix
    /// 
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            RunTestcase();
        }

        public static void RunTestcase()
        {
            var spiral = MatrixSpiralPrint(new int[][] { new int[]{ 1, 2, 3 }, new int[]{ 8, 9, 4 }, new int[]{ 7, 6, 5 } });
            Debug.Assert(String.Join("", spiral).CompareTo("123456789") == 0);
        }

        public static IList<int> MatrixSpiralPrint(int[][] matrix)
        {
            var result = new List<int>();

            if (matrix == null || matrix.Length == 0 || matrix[0].Length == 0)
            {
                return result;
            }

            helper(matrix, result, 0, 0, matrix.Length - 1, matrix[0].Length - 1);

            return result;
        }
       
        /// <summary>
        /// recursive function - 
        /// check base cases:
        /// out of array range
        /// one element
        /// one row
        /// one column
        /// 
        /// </summary>
        /// <param name="matrix"></param>
        /// <param name="result"></param>
        /// <param name="rowStart"></param>
        /// <param name="colStart"></param>
        /// <param name="rowEnd"></param>
        /// <param name="colEnd"></param>
        private static void helper(int[][] matrix, IList<int> result, int rowStart, int colStart, int rowEnd, int colEnd)
        {
            // stop situation
            if (rowStart > rowEnd || colStart > colEnd)
            {
                return;
            }

            var current = matrix[rowStart][colStart];

            // only one element in the sub matrix
            if (rowStart == rowEnd && colStart == colEnd)
            {
                result.Add(current);
                return;
            }

            // only one row in the sub matrix
            if (rowStart == rowEnd)
            {
                // add this row
                for (int col = colStart; col <= colEnd; col++)
                {
                    result.Add(matrix[rowStart][col]);
                }

                return;
            }

            // only one column in the sub matrix
            if (colStart == colEnd)
            {
                // add this column
                for (int row = rowStart; row <= rowEnd; row++)
                {
                    result.Add(matrix[row][colStart]);
                }

                return;
            }

            // other cases, recursive adding all
            // add first row
            for (int col = colStart; col < colEnd; col++)
            {
                result.Add(matrix[rowStart][col]);
            }

            // add last column
            for (int row = rowStart; row < rowEnd; row++)
            {
                result.Add(matrix[row][colEnd]);
            }

            // add last row
            for (int col = colEnd; col > colStart; col--)
            {
                result.Add(matrix[rowEnd][col]);
            }

            // add first column
            for (int row = rowEnd; row > rowStart; row--)
            {
                result.Add(matrix[row][colStart]);
            }

            // recursive adding sub matrix
            helper(matrix, result, rowStart + 1, colStart + 1, rowEnd - 1, colEnd - 1);
        }
    }
}