两次BFS，一次记录火在不同的时间蔓延的地方。第二次搜索最短路线。

#include <cstdio>
#include <iostream>
#include <cstring>
#include <queue>
#include <vector>
using namespace std;

struct Node{
    int x,y;
};

char map[1005][1005];
int fireMap[1005][1005],r[1005][1005];
Node JNode;
int R,C;
int dir[4][2] = {{1,0},{0,1},{-1,0},{0,-1}};

queue<Node> QF;


void fbfs(){
    while (!QF.empty()) QF.pop();
    Node fNode,FirstNode,NextNode;
    for(int i=0;i<R;i++)
        for(int j=0;j<C;j++)
            if(map[i][j]=='F'){
                fNode.x=i;fNode.y=j;
                QF.push(fNode);
                fireMap[fNode.x][fNode.y]=1;
            }
    while (!QF.empty()) {
        FirstNode = QF.front();
        QF.pop();
        for(int i=0;i<4;i++){
            NextNode.x=FirstNode.x+dir[i][0];
            NextNode.y=FirstNode.y+dir[i][1];

            if(NextNode.x < 0 || NextNode.x >= R || NextNode.y < 0 || NextNode.y >= C)
                continue;
            if(map[NextNode.x][NextNode.y]=='#')
                continue;
            if(fireMap[NextNode.x][NextNode.y]==0){
                fireMap[NextNode.x][NextNode.y]=fireMap[FirstNode.x][FirstNode.y]+1;
                QF.push(NextNode);
            }
        }
    }
    QF.push(JNode);
    r[JNode.x][JNode.y]=1;
    while (!QF.empty()) {
        FirstNode = QF.front();
        QF.pop();
        if(FirstNode.x==0 || FirstNode.x==R-1 || FirstNode.y==0 || FirstNode.y==C-1){
            printf("%d\n",r[FirstNode.x][FirstNode.y]);
            return;
        }

        for(int i=0;i<4;i++){
            Node Next;
            Next.x = FirstNode.x+dir[i][0];
            Next.y = FirstNode.y+dir[i][1];
            if(Next.x<0 || Next.x>=R || Next.y<0 || Next.y>=C) continue;
            if(map[Next.x][Next.y]=='#') continue;

            if(r[Next.x][Next.y]==0 && (r[FirstNode.x][FirstNode.y]+1 < fireMap[Next.x][Next.y] || fireMap[Next.x][Next.y]==0)){
                r[Next.x][Next.y]=r[FirstNode.x][FirstNode.y]+1;
                QF.push(Next);
            }
        }
    }
     puts("IMPOSSIBLE");
}


int main(){
// freopen("cin", "r", stdin);
// freopen("cout", "w", stdout);
    int c;
    scanf("%d",&c);
    while(c--){
        memset(r,0,sizeof(r));
        memset(fireMap,0,sizeof(fireMap));
        scanf("%d%d",&R,&C);
        for(int i=0;i<R;i++){
            scanf("%s",map[i]);
            for(int j=0;j<C;j++){
                if(map[i][j]=='J') {JNode.x=i;JNode.y=j;}
            }
        }
        fbfs();
    }
    return 0;
}