public class Solution {
    public int[] FindDiagonalOrder(int[][] mat)
    {
        if (mat == null || mat.Length == 0 || mat[0].Length == 0)
        {
            return new int[0];
        }

        var m = mat.Length;
        var n = mat[0].Length;

        var diagonals = new Dictionary<int, List<int>>();

        for (var i = 0; i < m; i++)
        {
            for (var j = 0; j < n; j++)
            {
                int diagonalIndex = i + j;
                if (!diagonals.ContainsKey(diagonalIndex))
                {
                    diagonals[diagonalIndex] = new List<int>();
                }
                diagonals[diagonalIndex].Add(mat[i][j]);
            }
        }

        var result = new List<int>();
        foreach (var key in diagonals.Keys)
        {
            var diagonal = diagonals[key];
            if (key % 2 == 0)
            {
                result.AddRange(diagonal);
            }
            else
            {
                diagonal.Reverse();
                result.AddRange(diagonal);
            }
        }

        return result.ToArray();
    }
}

class Program
{
    static void Main()
    {
        var Solution = new Solution();

        int[][] mat1 = new int[][]{
            new int[]{1,2,3},
            new int[]{4,5,6},
            new int[]{7,8,9}
        };

        Console.WriteLine(string.Join(", ", Solution.FindDiagonalOrder(mat1)));

    }
}