Problem:
Problem B: Red and Black

There is a rectangular room, covered with square tiles. Each tile is
colored either red or black. A man is standing on a black tile.
From a tile, he can move to one of four adjacent tiles. But he can't
move on red tiles, he can move only on black tiles.

Write a program to count the number of black tiles which he can reach
by repeating the moves described above.

Input

The input consists of multiple data sets.
A data set starts with a line containing two positive integers W and H;
W and H are the numbers of tiles in the x-
and y- directions, respectively. W and H are
not more than 20.

There are H more lines in the data set, each of which
includes W characters. Each character represents the color of a
tile as follows.

'.' - a black tile

'#' - a red tile

'@' - a man on a black tile(appears exactly once in a data set)

The end of the input is indicated by a line consisting of two zeros.

Output

For each data set, your program
should output a line which contains the number of
tiles he can reach from the initial tile (including itself).

Sample Input

6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

Output for the Sample Input

45
59
6
13

Solution in C# (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CompetitiveProg
{
    class Program
    {
        static int w, h;
        static int[,] room;
        static bool[,] roomVisited;
        static int count;
        static void Main(string[] args)
        {
            var ans = new List<int>();
            while(true)
            {
                count = 0;
                var wh = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
                if (wh[0] == 0 && wh[1] == 0) break;
                w = wh[0]; h = wh[1];
                room = new int[w, h];
                roomVisited = new bool[w, h];
                int[] start = new int[2];
                for (int j = 0; j < h; j++)
                {
                    var row = Console.ReadLine();
                    var i = 0;
                    foreach (var r in row)
                    {
                        switch (r)
                        {
                            case '.':
                                room[i, j] = 1;
                                break;
                            case '#':
                                room[i, j] = 0;
                                break;
                            case '@':
                                room[i, j] = 1;
                                start = new[] { i, j };
                                break;
                        }
                        i++;
                    }


                }
                for (int i = 0; i < w; i++)
                {
                    for (int j = 0; j < h; j++)
                    {
                        roomVisited[i, j] = false;
                    }
                }
                dfs(start[0], start[1]);
                ans.Add(count);
            }
            foreach (var c in ans)
            {
                Console.WriteLine(c);
            }
            

        }
        static void dfs(int startx,int starty)
        {
            if (roomVisited[startx, starty]) return;
            roomVisited[startx, starty] = true;
            count++;
            if ((startx - 1 >= 0) && room[startx - 1, starty]==1)
            {
                dfs(startx - 1, starty);
            }
            if ((starty - 1 >= 0)&& room[startx , starty-1]==1)
            {
                dfs(startx, starty-1);
            }
            if ((startx + 1<w)&& room[startx + 1, starty]==1)
            {
                dfs(startx + 1, starty);
            }
            if ((starty + 1 < h) && room[startx , starty+1] == 1)
            {
                dfs(startx, starty+1);
            }
        }
       
    }
}