Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

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

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

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])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include <iostream>
#include <algorithm>
using namespace std;
​
int W, H;
char visit[21][21];
int dfs(int h, int w){
  int sum=1;
  if(0>w || w==W || 0>h || h==H) return 0;
  if(visit[h][w]=='#')  return 0;
  if(visit[h][w]=='!')   return 0;
    visit[h][w] = '!';
    sum += dfs(h, w+1);
    sum += dfs(h, w-1);
    sum += dfs(h+1, w);
    sum += dfs(h-1, w);
  
  return sum;
}
​
int main(void){
  int w1, h1;
  string str;
  while(cin >> W >> H){
    if(W == 0 && H == 0)  break;
    for(int i=0; i<H; i++){
      cin >> str;
      for(int j=0; j<W; j++){
        visit[i][j] = str[j];
        if(visit[i][j] == '@'){
          visit[i][j] = '.';
          w1 = j; h1 = i;  
        }
      }
    }
    cout << dfs(h1, w1) <<endl;
  }
  
  return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <bitset>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<pint> vpint;
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for (int i=(n)-1;i>=0;i--)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--)
int dx[4] = {1, 0, 0, -1};
int dy[4] = {0, 1, -1, 0};

char board[21][21];
int w, h;
int x, y;// ?????¨?????????

void slove(void){
	int cnt = 0;
	queue<pint> q;
	q.push(make_pair(y, x));//?????¨????????????queue?????\??????

	while(!q.empty()){
		pint now = q.front(); q.pop();
		rep(i, 4){
			int nowy = now.fi + dy[i];
			int nowx = now.se + dx[i];
			if (0 <= nowy <= h - 1 && 0 <= nowx && nowx <= w - 1){
				if (board[nowy][nowx] == '.'){
					cnt++;
					printf("%d\n", cnt);
					board[nowy][nowx] = 0;
					q.push(make_pair(nowy, nowx));
				}
			}
		}
	}
	printf("%d\n", cnt + 1);//?§?????????´????????\??????
	return;
}

void scan(int w, int h){
	rep(i, h){
		rep(j, w){
			cin >> board[i][j];
			if (board[i][j] == '@'){
				y = i; x = j;
			}
		}
		getchar();
	}
	/*
	rep(i, h){
		rep(j, w){
			printf("%c", board[i][j]);
		}
		printf("\n");
	}
	*/
	slove();
	return;
}

int main(void){
	rep(i, 1000){
		cin >> w >> h;

		if (w == 0) break;
		scan(w, h);
	}
	return 0;
}

C++: #include <iostream>
#define MAX 22
using namespace std;

int counter = 0;
char tile[MAX][MAX];

void redtile(int y, int x){
	tile[y][x] = '+';
	counter++;
	if (x + 1 < MAX && tile[y][x + 1] == '.')
		redtile(y, x + 1);
	if (x - 1 >= 0 && tile[y][x - 1] == '.')
		redtile(y, x - 1);
	if (y + 1 < MAX&&tile[y + 1][x] == '.')
		redtile(y + 1, x);
	if (y - 1 >= 0 && tile[y - 1][x] == '.')
		redtile(y - 1, x);
}


int main()
{
	int w, h, initi, initj;
	while (cin >> w >> h, w || h){
		for (int i = 0; i < MAX; i++){
			for (int j = 0; j < MAX; j++){
				tile[i][j] = 0;
			}
		}
		for (int i = 0; i < h; i++){
			for (int j = 0; j < w; j++){
				cin >> tile[i][j];
				if (tile[i][j] == '@'){
					tile[i][j] = '.';
					initi = i;
					initj = j;
				}
			}
		}
		counter = 0;
		redtile(initi, initj);
		cout << counter << endl;
	}
	return 0;
}

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

int W, H;
char visit[21][21];
int dfs(int h, int w){
  int sum=1;
  if(0>w || w==W || 0>h || h==H) return 0;
  if(visit[h][w]=='#')  return 0;
  if(visit[h][w]=='!')   return 0;
    visit[h][w] = '!';
    sum += dfs(h, w+1);
    sum += dfs(h, w-1);
    sum += dfs(h+1, w);
    sum += dfs(h-1, w);
   
  return sum;
}

int main(void){
  int w1, h1;
  string str;
  while(cin >> W >> H){
    if(W == 0 && H == 0)  break;
    for(int i=0; i<H; i++){
      cin >> str;
      for(int j=0; j<W; j++){
        visit[i][j] = str[j];
        if(visit[i][j] == '@'){
          visit[i][j] = '.';
          w1 = j; h1 = i;  
        }
      }
    }
    cout << dfs(h1, w1) <<endl;
  }
   
  return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int ans;
int w,h;
string map[40];
int vec[4][2]={{0,1},{0,-1},{-1,0},{1,0}};
void move(int x,int y){
	ans++;
	map[x][y]='#';
	for(int i=0;i<4;i++){
		if((x+vec[i][0]>=0)&&(y+vec[i][1]>=0)&&(x+vec[i][0]<h)&&(y+vec[i][1]<w)){
			if(map[x+vec[i][0]][y+vec[i][1]]=='.'){
				move(x+vec[i][0],y+vec[i][1]);
			}
		}
	}
	return;
}

int main(){
	while(cin>>w>>h,w){
		ans=0;
		int fx,fy;
		for(int i=0;i<h;i++){
			cin>>map[i];
		}
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				if(map[i][j]=='@'){
					fx=i;
					fy=j;
					break;
				}
			}
		}
		move(fx,fy);
		cout<<ans<<endl;
		
	}
}