为什么WA

#include <bits/stdc++.h>
#define re register
using namespace std;
const int N = 505;
char graph[N][N];
int n, m, cnt, dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, -1, 1};
inline int read(){
    int f = 0, x = 0; char ch;
    do {ch = getchar(); f |= ch == '-';} while (!isdigit(ch));
    do {x = (x << 3) + (x << 1) + (ch ^ 48); ch = getchar();} while (isdigit(ch));
    return f ? -x : x;
}
inline bool check(int x, int y){
    return x > 0 && x <= n && y > 0 && y <= m;
}
inline void dfs1(int x, int y){
    for (re int i = 0; i < 4; ++i){
        int nx = x + dx[i], ny = y + dy[i];
        if (check(nx, ny) && graph[nx][ny] != '*'){
            graph[nx][ny] = '*';
            dfs1(nx, ny);
        }
    }
}
inline void dfs2(int x, int y){
    for (re int i = 0; i < 4; ++i){
        int nx = x + dx[i], ny = y + dy[i];
        if (check(nx, ny) && graph[nx][ny] != '*'){
            graph[nx][ny] = '*';
            dfs2(nx, ny);
        }
    }
}
int main(){
	//freopen("data.txt", "r", stdin);
    n = read(), m = read();
    for (re int i = 1; i <= n; ++i)scanf("%s", graph[i] + 1);
    for (re int i = 1; i <= n; ++i){
        if (graph[i][1] == '0'){
        	graph[i][1] = '*';
        	dfs1(i, 1);
		}
        if (graph[1][i] == '0'){
        	graph[1][i] = '*';
        	dfs1(1, i);
		}
        if (graph[n][i] == '0'){
        	graph[n][i] = '*';
			dfs1(n, i);
		}
        if (graph[i][n] == '0'){
        	graph[i][n] = '*';
        	dfs1(i, n);
		}
    }
    for (re int i = 1; i <= n; ++i){
        for (re int j = 1; j <= m; ++j){
            if (graph[i][j] == '0'){
            	++cnt;
                dfs2(i, j);
            }
        }
    }
    printf("%d", cnt);
    return 0;
}