DepthFirstSearch

using System;
class DepthFirstSearch
{
    static void Main()
    {
        int[,] m = { { 1, 3, 2, 2, 2, 4 }, { 3, 3, 3, 2, 4, 4 }, { 4, 3, 1, 2, 3, 3 }, { 4, 3, 1, 3, 3, 1 } };
        int maxPath = 0;
        for (int r = 0; r < m.GetLength(0); r++)
            for (int c = 0; c < m.GetLength(1); c++)
            {
                int count = 1;
                FindPath(m, m[r, c], new bool[m.GetLength(0), m.GetLength(1)], r, c, ref count);
                if (count > maxPath) maxPath = count;
            }
        Console.WriteLine(maxPath);
    }
    private static void FindPath(int[,] m, int n, bool[,] tM, int r, int c, ref int count)
    {
        tM[r, c] = true;
        if (r > 0 && !tM[r - 1, c] && m[r - 1, c] == n) count = Count(m, n, tM, count, r - 1, c);
        if (r < m.GetLength(0) - 1 && !tM[r + 1, c] && m[r + 1, c] == n) count = Count(m, n, tM, count, r + 1, c);
        if (c > 0 && !tM[r, c - 1] && m[r, c - 1] == n) count = Count(m, n, tM, count, r, c - 1);
        if (c < m.GetLength(1) - 1 && !tM[r, c + 1] && m[r, c + 1] == n) count = Count(m, n, tM, count, r, c + 1);
    }
    private static int Count(int[,] m, int n, bool[,] tM, int count, int nr, int nc)
    {
        tM[nr, nc] = true;
        count++;
        FindPath(m, n, tM, nr, nc, ref count);
        return count;
    }
}