先用bfs跑一遍火和人走到每一格的最短时间 最后再找人能走到而且火没跑到的最短边缘 AC代码:

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1010;
struct node {
    int x;int y;
};
char mp[N][N];
int f[N][N];
int q[N][N];
int xlj[4]={1,-1,0,0};
int ylj[4]={0,0,1,-1};
int n;int m;
void bfs(int st[N][N],queue<node>&qqq) {
    while (!qqq.empty()) {
        node t=qqq.front();qqq.pop();
        for (int i=0;i<4;i++) {
            int x=t.x+xlj[i];
            int y=t.y+ylj[i];
            if (mp[x][y]=='#')continue;
            if (st[x][y]!=-1)continue;
            if (x<1||x>n||y<1||y>m)continue;
            qqq.push({x,y});
            st[x][y]=st[t.x][t.y]+1;
        }
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin>>n>>m;
    queue<node>fq;
    queue<node>qq;
    for (int i=1;i<=n;i++) {
        string s;cin>>s;
        for (int j=1;j<=m;j++) {
            mp[i][j]=s[j-1];
            f[i][j]=-1;
            q[i][j]=-1;
            if (mp[i][j]=='F') {
                fq.push({i,j});
                f[i][j]=0;
            }
            if (mp[i][j]=='J') {
                qq.push({i,j});
                q[i][j]=0;
            }
        }
    }
    bfs(q,qq);
    bfs(f,fq);
    for (int i=1;i<=n;i++)qq.push({i,1});
    for (int i=1;i<=m;i++)qq.push({1,i});
    for (int i=1;i<=n;i++)qq.push({i,m});
    for (int i=1;i<=m;i++)qq.push({n,i});
    int ans=INT_MAX;
    while (!qq.empty()) {
        node t=qq.front();qq.pop();
        int x=t.x;int y=t.y;
        if (q[x][y]!=-1&&(q[x][y]<f[x][y]||f[x][y]==-1)) {
            ans=min(q[x][y],ans);
        }
    }
    if (ans==INT_MAX) cout<<"IMPOSSIBLE";
    else cout<<ans+1;
    return 0;
}