有个人在一块区域内, 这块区域某个地方着火了, 问你这个人能否安全逃离这个区域, 人跟火蔓延的速度都是一样的, 这块区域可能有多个地方同时着火.

用两次 bfs, 第一次标记火蔓延, 第二次求人到达出口的最短时间.

#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int N = 1005;
int dx[] = {0, 0, -1, 1};
int dy[] = {1, -1, 0, 0};
char mp[N][N];
int fire[N][N], walk[N][N];
int sx, sy;
int c, r;
void bfs1()
{
    memset(fire, INF, sizeof(fire));
    queue<pair<int, int> > que;
    for (int i = 0; i < r; i++) {
        for (int j = 0; j < c; j++) {
            if (mp[i][j] == 'F') {
                fire[i][j] = 0;
                que.push(make_pair(i, j));
            }
        }
    }
    while (!que.empty()) {
        auto u = que.front();
        que.pop();
        for (int i = 0; i < 4; i++) {
            int nx = u.first + dx[i], ny = u.second + dy[i];
            if (0 <= nx && nx < r && 0 <= ny && ny < c) {
                if (mp[nx][ny] == '#' || fire[nx][ny] != INF) {
                    continue;
                }
                fire[nx][ny] = fire[u.first][u.second] + 1;
                que.push(make_pair(nx, ny));
            }
        }
    }
}
int bfs2()
{
    memset(walk, -1, sizeof(walk));
    queue<pair<int, int> > que;
    que.push(make_pair(sx, sy));
    walk[sx][sy] = 0;
    while (!que.empty()) {
        auto u = que.front();
        que.pop();
        for (int i = 0; i < 4; i++) {
            int nx = u.first + dx[i], ny = u.second + dy[i];
            if (nx < 0 || nx >= r || ny < 0 || ny >= c) {
                return walk[u.first][u.second] + 1;
            }
            if (mp[nx][ny] == '#' || walk[nx][ny] != -1)
                continue;
            if (walk[u.first][u.second] + 1 >= fire[nx][ny])
                continue;
            walk[nx][ny] = walk[u.first][u.second] + 1;
            que.push(make_pair(nx, ny));
        }
    }
    return -1;
}
int main()
{
    int t;
    scanf("%d", &t);
    while (t--) {
        scanf("%d %d", &r, &c);
        for (int i = 0; i < r; i++) {
            scanf("%s", mp[i]);
            for (int j = 0; j < c; j++) {
                if (mp[i][j] == 'J') {
                    sx = i, sy = j;
                }
            }
        }
        bfs1();
        int ans = bfs2();
        if (ans == -1) {
            puts("IMPOSSIBLE");
        } else {
            printf("%d\n", ans);
        }
    }
    return 0;
}

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