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 PHP (judge verdict: Accepted)

<?php

function c_p($i,$j){
    global $sum,$a;
    $sum+=1;
    $a[$i][$j]="#";
}

function search($i,$j){
    global $a;
    if(isset($a[$i+1][$j]) and $a[$i+1][$j]=="."){
        c_p($i+1,$j);
        search($i+1,$j);
    }
    if(isset($a[$i-1][$j]) and $a[$i-1][$j]=="."){
        c_p($i-1,$j);
        search($i-1,$j);
    }
    if(isset($a[$i][$j+1]) and $a[$i][$j+1]=="."){
        c_p($i,$j+1);
        search($i,$j+1);
    }
    if(isset($a[$i][$j-1]) and $a[$i][$j-1]=="."){
        c_p($i,$j-1);
        search($i,$j-1);
    }
}


while(1){
    $a = array();
    $sum=1;
    fscanf(STDIN,"%d %d\n",$w,$h);

    if($w==0 && $h==0)
        exit;
    
    for($i=0;$i<$h;$i++){
        fscanf(STDIN,"%s\n",$a[$i]);
        $a[$i] = str_split($a[$i]);
    }
    
//print_r($a);
    for($i=0;$i<$h;$i++){
        for($j=0;$j<$w;$j++){       
            if($a[$i][$j]=="@"){
                $row = $i;
                $col = $j;
            }
        }
    }
    
    $sum=1;
    
    search($row,$col);
    echo $sum."\n";
}


?>