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())
	}
}

Python: #!/usr/bin/env python3
#AOJ B
from collections import deque

def bfs():
    while 1:
        W,H = map(int,input().split())
        if W == 0 and H == 0:
            break
        maze = [list(input()) for _ in range(H)]
        check = [[False for _ in range(W)] for _ in range(H)]
        #print(maze)
        dx = [1,0,-1,0]
        dy = [0,1,0,-1]
        for i in range(H):
            for j in range(W):
                if maze[i][j] == '@':
                    sx,sy = j,i
                    check[i][j] = True

        ans = 1
        que = deque()
        que.append((sx,sy))
        while que:
            x,y = que.pop()
            for i in range(4):
                nx,ny = x + dx[i],y + dy[i]

                if 0 <= nx < W and 0 <= ny < H and maze[ny][nx] == '.' and check[ny][nx] == False:
                    que.append((nx,ny))
                    check[ny][nx] = True
                    ans += 1
        print(ans)
    return

if __name__ == '__main__':
    bfs()

Python: while True:
    w, h = map(int, input().split())
    if h == 0:
        break
    m = [list(input()) for _ in range(h)]
    sx = 0
    sy = 0
    for yi, row in enumerate(m):
        if '@' in row:
            sy = yi
            sx = row.index('@')
    def dfs(x, y):
        if x < 0 or w <= x or y < 0 or h <= y or m[y][x] == '#':
            return 0
        else:
            m[y][x] = '#'
            dy = [1, -1] + [0] * 2
            dx = [0] * 2 + [1, -1]
            return sum([dfs(x+dx[i], y+dy[i]) for i in range(4)]) + 1
    print(dfs(sx, sy))

C++: #include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cassert>
#include <string>
#include <algorithm>
#include <vector>
#include <valarray>
#include <queue>
#include <stack>
#include <functional>
#include <iostream>
#include <iomanip>
#include <map>
#include <set>
#include <list>
#include <limits>
#include <numeric>
#include <utility>
#include <deque>
#include <unordered_set>
#include <unordered_map>
#include <regex>
#include <array>
#include <bitset>
#include <complex>
#include <tuple>
#include <random>

using namespace std;
using ll = long long;
const int INF = 1e9;
const ll MOD = 1e9 + 7;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

void dfs(int x, int y, int &sum, vector<string> &field)
{
  field[y][x] = '#';
  sum++;
  for (int i = 0; i < 4; ++i)
  {
    int ny = y + dy[i], nx = x + dx[i];
    if (0 <= ny && ny < field.size() && 0 <= nx && nx < field[0].size() && field[ny][nx] == '.')
    {
      dfs(nx, ny, sum, field);
    }
  }
}

int main()
{
  int w, h;
  while (cin >> w >> h, w || h)
  {
    vector<string> field(h);
    for (auto &e : field)
      cin >> e;
    int sx, sy;
    for (int i = 0; i < h; ++i)
    {
      for (int j = 0; j < w; ++j)
      {
        if (field[i][j] == '@')
        {
          sy = i, sx = j;
          field[i][j] = '#';
        }
      }
    }
    int sum = 0;
    dfs(sx, sy, sum, field);
    cout << sum << endl;
  }

  return 0;
}

C++: //red and black
#if 0

#endif // 0
#include<iostream>
#include<queue>
#include<stack>
using namespace std;
#define MAX 1000
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
int matri[MAX][MAX],s[MAX][MAX];
int w,h;
bool valid(int x,int y)
{
    return x>=0&&x<w&&y>=0&&y<h&&matri[x][y];
}
int main()
{
    while(cin>>w>>h&&w&&h)
    {
        pair<int,int> p;
        for(int j=0;j<h;j++)
            for(int i=0;i<w;i++)
            {
                s[i][j]=1;
                char c;
                cin>>c;
                if(c=='.')
                    matri[i][j]=1;
                else if(c=='#')
                {
                    matri[i][j]=0;
                    s[i][j]=0;
                }
                else if(c=='@'){
                    matri[i][j]=1;
                    p.first=i;
                    p.second=j;
                }
            }
        stack<pair<int,int> > q;
        int sum=0;
        q.push(p);
        s[p.first][p.second]=0;
        while(!q.empty())
        {
            p=q.top();
            int i;
            for(i=0;i<4;i++)
            {
                int x=p.first+dx[i];
                int y=p.second+dy[i];
                if(valid(x,y)&&s[x][y])
                {
                    p.first=x;
                    p.second=y;
                    q.push(p);
                    s[x][y]=0;
                    break;
                }
            }
            if(i>=4)
            {
                sum++;
                q.pop();
            }
        }
        cout<<sum<<endl;
    }
    return 0;
}