Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

type Node struct {
	ID    int
	X, Y  int
	Color Color
	Nexts []*Node
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		// Initialize map
		m := map[int]map[int]*Node{}
		for x := 1; x <= w; x++ {
			m[x] = map[int]*Node{}
			for y := 1; y <= h; y++ {
				m[x][y] = &Node{ID: x + y*w, X: x, Y: y}
			}
		}
		for x := 1; x <= w; x++ {
			for y := 1; y <= h; y++ {
				nexts := []*Node{}
				if n, ok := m[x-1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y-1]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x+1][y]; ok {
					nexts = append(nexts, n)
				}
				if n, ok := m[x][y+1]; ok {
					nexts = append(nexts, n)
				}
				m[x][y].Nexts = nexts
			}
		}

		// Set colors
		queue := []*Node{}
		touchedIDs := map[int]struct{}{}
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					n := m[i+1][y]
					n.Color = ColorBlack
					if c == '@' {
						queue = append(queue, n)
						touchedIDs[n.ID] = struct{}{}
					}
				case '#':
					m[i+1][y].Color = ColorRed
				}
			}
		}

		// Search
		visitedIDs := map[int]struct{}{}
		for {
			if len(queue) == 0 {
				break
			}
			n := queue[0]
			queue = queue[1:]
			visitedIDs[n.ID] = struct{}{}
			for _, next := range n.Nexts {
				if next.Color == ColorRed {
					continue
				}
				if _, ok := touchedIDs[next.ID]; !ok {
					queue = append(queue, next)
					touchedIDs[n.ID] = struct{}{}
				}
			}
		}
		fmt.Println(len(visitedIDs))
	}
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;

typedef  vector< vector<int> > vector2d;

int main() {
  
    int H,W,count=0,kaisuu=0,kosuu,sum,judge;
    char map[20][20];
    vector2d nextx,nexty; //nextx[回数][個数]
    nextx.reserve(1000);
    nexty.reserve(1000);

  while(1){
    cin >>W>>H;
    if (W==0&&H==0)break;
    
    //マップの入力
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cin >> map[cW][cH];
	  if (map[cW][cH]=='@'){
	    nextx[kaisuu].push_back(cW);
	    nexty[kaisuu].push_back(cH);
	  }
	}
    }
    sum = 1;
    while(1){
      judge = 0;
      for(kosuu=0;kosuu<nextx[kaisuu].size();kosuu++){
	if (nextx[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の左側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]-1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]-1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]-1!=-1){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]=='.'){  //@の上側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]-1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]-1 ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nextx[kaisuu][kosuu]+1!=W){
	  if(map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]=='.'){  //@の右側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]+1);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]);
	    map[ nextx[kaisuu][kosuu]+1 ][ nexty[kaisuu][kosuu] ]='@';
	    sum++;
	    judge++;
	  }
	}
	if (nexty[kaisuu][kosuu]+1!=H){
	  if(map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]=='.'){  //@の下側をチェック
	    //もし'@'だったらその座標を保存しそれを@に！
	    nextx[kaisuu+1].push_back(nextx[kaisuu][kosuu]);
	    nexty[kaisuu+1].push_back(nexty[kaisuu][kosuu]+1);
	    map[ nextx[kaisuu][kosuu] ][ nexty[kaisuu][kosuu]+1 ]='@';
	    sum++;
	    judge++;
	  }
	}
      }
      if (judge==0)break;
      kaisuu++;
    }
    
    cout << sum <<endl;
    
    for(int cH=0;cH<H;cH++){
	for(int cW=0;cW<W;cW++){
	  cout << map[cW][cH];
	}
	cout << endl;
    }
  }
  
  return 0;
  
}

C++: #include<iostream>
#include<algorithm>
#include<utility>
#include<queue>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
#define P pair<int, int>

using namespace std;
char field[20][20];
int W, H,sx,sy,nx,ny;
int dx[4] = { 1, 0, -1, 0 };
int dy[4] = { 0, 1, 0, -1 };
int cnt = 1;


void bfs(){
	queue<P> que;
	que.push(P(sx, sy));

	while (1){

		if (que.empty()){
			break;
		}
		P p = que.front();
		que.pop();
		
		rep(i, 4){
			nx = p.first + dx[i];
			ny = p.second + dy[i];
			if (nx >= 0 && nx < W && 0 <= ny && ny < H && field[ny][nx] == '.'){
				field[ny][nx] = '#';
				que.push(P(nx, ny));
				cnt++;
			}

		}

		/*

		rep(i, H){
			rep(j, W){
				cout << field[i][j];
			}
			cout << endl;
		}

		*/
	}



}

C++: #include <stdio.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cctype>

#include <iostream>
#include <vector>
#include <deque>
#include <queue>
#include <map>
#include <algorithm>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i,l,n) for(int (i)=(int)(l);(i)<(int)(n);(i)++)
#define d_arr(arr, h, w) rep(i,(h) ){ cout << "["; rep(j,(w) ) { cout << (arr)[i][j] << ", "; } cout << "]" << endl;}
#define IN(x,s,g) ((x) >= (s) && (x) < (g))
#define ISIN(x,y,w,h) (IN((x),0,(w)) && IN((y),0,(h)))
#define print(x) printf("%d\n",x);
#define fs first
#define sc second

typedef pair<int ,int> P;

char grid[20][20];
bool arrived[20][20];
int dx[4] ={1,0,-1,0};
int dy[4] ={0,1,0,-1};
int w,h;

//i,jから初めて到達可能なタイルの数の最大値(i,j)含む
void rec(int i,int j){
//    printf("i:%d j:%d\n",i,j);
    arrived[i][j] = true;
    grid[i][j] = '#';
    int ret = 0;
    rep(k,4) {
        int next_j = j+dx[k];
        int next_i = i+dy[k];
        if( next_j < w && 0 <= next_j &&
            next_i < h && 0 <= next_i &&
            grid[next_i][next_j] == '.')
        {
            rec(next_i,next_j);
        }
    }
}

int main()
{
    P start;
    while(1){
        cin >> w >> h;
        rep(i,20) rep(j,20) arrived[i][j] = false;
        rep(i,h){
            rep(j,w){
                cin >> grid[i][j];
                if( grid[i][j] == '@' ){ start.first=i;start.second=j; }
            }
        }
        rec(start.first,start.second);
        int ret=0;
        rep(i,20){
            rep(j,20){
                if(arrived[i][j] == true ){
                    ret+=1;
                }
            }
        }
        cout << ret << endl;
    }
}

C++: #include<iostream>
#include<queue>
#include<string>
#define N 20
using namespace std;

int main(){
	int w, h;
	while(cin >>w >>h){
		if(w == 0 && h == 0) break;
		int cnt = 1, dxh[4] = {-1, 0, 1, 0}, dxw[4] = {0, 1, 0, -1};
		string home[N];
		queue<int> quh, quw;
		for(int i = 0; i < h; i++){
			cin >>home[i];
			for(int j = 0; j < w; j++){
				if(home[i][j] == '@'){
					 quh.push(i);
					 quw.push(j);
				}
			}
		}
		while(quh.empty() != 1){
			int nowh = quh.front(), noww = quw.front();
			quh.pop();
			quw.pop();
			for(int i = 0; i < 4; i++){
				if(nowh + dxh[i] >= h || noww + dxw[i] >= w || nowh + dxh[i] < 0 || noww + dxw[i] < 0){
					continue;
				}
				if(home[nowh + dxh[i]][noww + dxw[i]] == '.'){
					quh.push(nowh + dxh[i]);
					quw.push(noww + dxw[i]);
					cnt++;
					home[nowh + dxh[i]][noww + dxw[i]] = '#';
				}
			}
		}
		cout <<cnt <<endl;
	}
	return 0;
}