【0-1 BFS】移除不必要的逻辑

```cpp

#include <bits/stdc++.h>

using namespace std;

#define INF (1 << 29)

int n, m;

char grid[1001][1001];

int dist[1001][1001][4];

int vis[1001][1001][4];

int fx[] = {1, -1, 0, 0};

int fy[] = {0, 0, 1, -1};

deque<int> q;

void add_front(int x, int y, int dir, int d) {

  if (d < dist[x][y][dir]) {

    dist[x][y][dir] = d;

    q.push_front(x);

  }

}

void add_back(int x, int y, int dir, int d) {

  if (d < dist[x][y][dir]) {

    dist[x][y][dir] = d;

    q.push_back(dir);

  }

}

int main() {

  cin >> n >> m;

  for (int i = 0; i < n; i++) cin >> grid[i];

    q.pop_front();

    q.pop_front();

    q.pop_front();

    if (vis[x][y][dir]) continue;

    vis[x][y][dir] = true;

    int d = dist[x][y][dir];

    int nx = x + fx[dir], ny = y + fy[dir];

    if (nx >= 0 && nx < n && ny >= 0 && ny < m) add_front(nx, ny, dir, d);