我用测试点给的数据来测试然后和答案对比发现一模一样，这为什么wa了？？？

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 1000 + 10;
int vis[MAXN][MAXN];//迷宫地图
bool is_vis[MAXN][MAXN]; //标记是否走过
int n, m;//n * n矩阵 m个问题
struct pos{
    int x;
    int y;
};
pos Div[4] = {{1 , 0}, {-1, 0}, {0, 1}, {0, -1}};//代表的前后左右移动

bool check(pos begin, pos Next){//检查有无越界已经能否移动(题目要求vis不同可以移动)
    if(Next.x > 0  && Next.x <= n  && Next.y > 0  && Next.y <= n)
        return  ((vis[begin.x][begin.y] != vis[Next.x][Next.y]) && (!is_vis[Next.x][Next.y]));
    else
        return false;  
}

int BFS(pos begin){
    int cnt = 1;//起点也算一点
    queue<pos> q;
    q.push(begin);
    is_vis[begin.x][begin.y] = true;
    pos One, Next;
    while(!q.empty()){
        One = q.front();
        q.pop();
        for(int i = 0; i < 4; i++){//前后左右四种选择
            Next.x = One.x + Div[i].x;
            Next.y = One.y + Div[i].y;
            if(check(One, Next)){
                //cout << Next.x << " " << Next.y << endl;
                q.push(Next);
                is_vis[Next.x][Next.y] = true;
                cnt++;
            }
        }
    }
    return cnt;
}

int main(){
    char c;
    scanf("%d %d", &n, &m);
    getchar();
    for(int i = 1; i <= n; i++){
        for(int j = 1; j <= n; j++){
            c = getchar();
            vis[i][j] = c - '0';
        }
        getchar();
    }
    for(int i = 1; i <= m; i++){
        pos start;
        scanf("%d %d",&start.x, &start.y);
        memset(is_vis, false, sizeof(is_vis));
        printf("%d\n", BFS(start));
    }
    return 0;
}