学步园

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <cstring>
#include <queue>
using namespace std;
#define INF 11000000
const int maxn = 1110;
char maze[maxn][maxn];
int n,m,Time[maxn][maxn],vis[maxn][maxn],sx,sy,fx[maxn],fy[maxn],degf,dist[maxn][maxn];
struct node{
int x,y;
node(int x=0,int y=0):x(x),y(y){}
};
const int dx[]={0,1,0,-1};
const int dy[]={1,0,-1,0};
bool judge(int x,int y){
return x>=0&&x<n&&y>=0&&y<m;
}
void bfs_firetime(){
queue<node> q;
for(int i=0;i<degf;i++){
        q.push(node(fx[i],fy[i]));
        dist[fx[i]][fy[i]]=0;
}
while(!q.empty()){
    node& u=q.front(); q.pop();
    for(int d=0;d<4;d++){
        int nx=dx[d]+u.x,ny=dy[d]+u.y;
        if(judge(nx,ny)&&maze[nx][ny]!='#'){
              if(dist[nx][ny]>dist[u.x][u.y]+1){
                  dist[nx][ny]=dist[u.x][u.y]+1;
                  q.push(node(nx,ny));
              }
        }
    }
}
}
int bfs(){
memset(vis,0,sizeof(vis));
queue<node> q;
q.push(node(sx,sy));
Time[sx][sy]=0; vis[sx][sy]=1;
if(sx==0||sy==0||sx==n-1||sy==m-1) return 1;
while(!q.empty()){
    node& u=q.front(); q.pop();
    for(int d=0;d<4;d++){
        int nx=dx[d]+u.x,ny=dy[d]+u.y;
        if(judge(nx,ny)&&maze[nx][ny]!='#'&&!vis[nx][ny]){
              if(dist[nx][ny]>Time[u.x][u.y]+1){
                  vis[nx][ny]=1;
                  Time[nx][ny]=Time[u.x][u.y]+1;
                  if(nx==0||ny==0||nx==n-1||ny==m-1) {
                  return Time[nx][ny]+1;
                  }
                  q.push(node(nx,ny));
              }
        }
    }
}
return -1;
}
int main()
{
    int T;
    scanf("%d",&T);
    while(T--){
        scanf("%d %d",&n,&m);
        degf=0;
        for(int i=0;i<n;i++){
            scanf("%s",maze[i]);
            for(int j=0;j<m;j++){
                 if(maze[i][j]=='J'){sx=i; sy=j;}
                 if(maze[i][j]=='F'){fx[degf]=i; fy[degf++]=j;}
            }
        }
        for(int i=0;i<=n;i++) for(int j=0;j<=m;j++) dist[i][j]=INF;
        bfs_firetime();
        int ans=bfs();
        if(ans>0) printf("%d\n",ans);
        else printf("IMPOSSIBLE\n");
    }
    return 0;
}