2257. Count Unguarded Cells in the Grid

public class Solution
{
    public int CountUnguarded(int m, int n, int[][] guards, int[][] walls)
    {
        int ans = 0;
        char[,] grid = new char[m, n];
        char[,] left = new char[m, n];
        char[,] right = new char[m, n];
        char[,] up = new char[m, n];
        char[,] down = new char[m, n];

        foreach (var guard in guards)
            grid[guard[0], guard[1]] = 'G';

        foreach (var wall in walls)
            grid[wall[0], wall[1]] = 'W';

        for (int i = 0; i < m; ++i)
        {
            char lastCell = '\0';
            for (int j = 0; j < n; ++j)
            {
                if (grid[i, j] == 'G' || grid[i, j] == 'W')
                    lastCell = grid[i, j];
                else
                    left[i, j] = lastCell;
            }

            lastCell = '\0';

            for (int j = n - 1; j >= 0; --j)
            {
                if (grid[i, j] == 'G' || grid[i, j] == 'W')
                    lastCell = grid[i, j];
                else
                    right[i, j] = lastCell;
            }
        }

        for (int j = 0; j < n; ++j)
        {
            char lastCell = '\0';
            for (int i = 0; i < m; ++i)
            {
                if (grid[i, j] == 'G' || grid[i, j] == 'W')
                    lastCell = grid[i, j];
                else
                    up[i, j] = lastCell;
            }

            lastCell = '\0';

            for (int i = m - 1; i >= 0; --i)
            {
                if (grid[i, j] == 'G' || grid[i, j] == 'W')
                    lastCell = grid[i, j];
                else
                    down[i, j] = lastCell;
            }
        }

        for (int i = 0; i < m; ++i)
        {
            for (int j = 0; j < n; ++j)
            {
                if (grid[i, j] == '\0' && left[i, j] != 'G' && right[i, j] != 'G' && up[i, j] != 'G'
                    && down[i, j] != 'G')
                    ++ans;
            }
        }

        return ans;
    }
}