```cpp
#include <iostream>
#include <queue>
using namespace std;
const int N = 1050;
int n, sx, sy, fx, fy, vis[N][N];
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};
char a[N][N];
int bfs(int posx, int posy)
{
queue <pair<pair<int, int>, int> > q;
q.push(make_pair(make_pair(posx, posy), 0));
while(!q.empty())
{
int x = q.front().first.first, y = q.front().first.second, round = q.front().second;
q.pop();
vis[x][y] = 1;
for(int i = 0; i < 4; i++)
{
int nx = x + dx[i], ny = y + dy[i];
if(nx < 1 || ny < 1 || nx > n || ny > n || vis[nx][ny] || a[nx][ny] == '1') continue;
if(nx == fx && ny == fy) return round + 1;
q.push(make_pair(make_pair(nx, ny), round + 1));
}
}
return -1;
}
int main()
{
cin >> n;
for(int i = 1; i <= n; i++)
for(int j = 1; j <= n; j++)
cin >> a[i][j];
cin >> sx >> sy >> fx >> fy;
cout << bfs(sx, sy) << endl;
return 0;
}
```
第 $28$ 行的 `return -1;` 是为了函数严谨，会终止并返回错误值，但是问题肯定不在这里。
by 159号程序员 @ 2022-01-18 13:56:10
@[159号程序员](/user/334586) 如果出队的时候标记，那么就可能会导致同一个东西多次入队，应该是这里错了
by fjy666 @ 2022-01-18 13:59:38
@[fjy666](/user/366338) 把 `vis` 标记移到循环里？
by 159号程序员 @ 2022-01-18 14:00:44
@[159号程序员](/user/334586) [Here.你是不是错了细节第二条？](https://www.luogu.com.cn/blog/Cube-Dasher/solution-p4328)
by Argon_Cube @ 2022-01-18 14:02:20
@[159号程序员](/user/334586) 就是在进队的时候 vis[x][y]=1 就行了qwq
by fjy666 @ 2022-01-18 14:03:23
@[Epsilon_Cube](/user/372983) 大概率是，我调一下看看
by 159号程序员 @ 2022-01-18 14:03:47
@[fjy666](/user/366338) 我试试看
by 159号程序员 @ 2022-01-18 14:04:04
```diff
while(!q.empty())
{
int x = q.front().first.first, y = q.front().first.second, round = q.front().second;
q.pop();
+ if (vis[x][y]) continue;
vis[x][y] = 1;
for(int i = 0; i < 4; i++)
{
int nx = x + dx[i], ny = y + dy[i];
- if(nx < 1 || ny < 1 || nx > n || ny > n || vis[nx][ny] || a[nx][ny] == '1') continue;
+ if(nx < 1 || ny < 1 || nx > n || ny > n || a[nx][ny] == '1') continue;
if(nx == fx && ny == fy) return round + 1;
q.push(make_pair(make_pair(nx, ny), round + 1));
}
}
```
by ud2_ @ 2022-01-18 14:04:17
@[ud2_](/user/206953) 理解
by 159号程序员 @ 2022-01-18 14:05:39
@[fjy666](/user/366338) @[ud2_](/user/206953) @[Epsilon_Cube](/user/372983)
感谢各位，AC了/qq
by 159号程序员 @ 2022-01-18 14:18:16