uva 11624 Fire!（多源BFS）

uva 11624 Fire!

题目大意：J在迷宫里工作，有一天迷宫起火了，火源有多处。每过一秒，火源都会向四个方向蔓延，J也会向四个方向移动，问J能不能跑出去，能的话输出他跑出去的最短时间，否则输出“IMPOSSIBLE”

解题思路：先进行一次BFS，找出每一点火焰蔓延到该处的最短时间。然后根据这张“火势图”，对J的行进路线进行BFS。注意J在边缘的时候，以及没有火源的时候。

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <cstdlib>
#include <queue>
using namespace std;

const int N = 1005;
typedef long long ll;
int n, m;
int dir[4][2] = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
struct Node{
    int x, y, d;
    bool operator < (const Node& x) const {
        return d > x.d; 
    }
};
Node s;
int cnt;
int fire[N][N];
queue<Node> Q;
priority_queue<Node> Q2;
char map[N][N];
int BFS_F() {
    while (!Q.empty()) {
        Node p = Q.front();
        Q.pop();    
        int px, py;
        for (int i = 0; i < 4; i++) {
            px = p.x + dir[i][0], py = p.y + dir[i][1];
            if (px < 0 || py < 0 || px >= n || py >= m) continue;
            if (map[px][py] == ‘#‘) continue;
            map[px][py] = ‘F‘;
            if (fire[px][py] > p.d + 1 || fire[px][py] == -1) {
                fire[px][py] = p.d + 1;
                Q.push((Node){px, py, p.d + 1});
            }
        }
    }   
}
int BFS() {
    while (!Q2.empty()) {
        Node p = Q2.top();
        Q2.pop();
        for (int i = 0; i < 4; i++) {
            int px = p.x + dir[i][0], py = p.y + dir[i][1];
            if (px < 0 || py < 0 || px >= n || py >= m) continue;
            if (map[px][py] == ‘#‘ || map[px][py] == ‘J‘) continue;
            if (fire[px][py] >= p.d + 2) {
                if (px == 0 || py == 0 || px == n - 1 || py == m - 1) {
                    return p.d + 1;
                }
                map[px][py] = ‘J‘;
                Q2.push((Node){px, py, p.d + 1});
            }
        }
    }
    return 0;
}   

int main() {
    int T;
    scanf("%d", &T);
    while (T--) {
        cnt = 0;
        memset(fire, -1, sizeof(fire));
        while (!Q.empty()) Q.pop();
        while (!Q2.empty()) Q2.pop();
        scanf("%d %d\n", &n, &m);   
        int flag = 0, flag2 = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                scanf("%c", &map[i][j]);    
                if (map[i][j] == ‘J‘) {
                    if (i == 0 || j == 0 || i == n - 1 || j == m - 1) {
                        flag = 1;
                    }
                    Q2.push((Node){i, j, 0});   
                } else if (map[i][j] == ‘F‘) {
                    flag2 = 1;
                    fire[i][j] = 0;
                    Q.push((Node){i, j, 0});
                }
            }
            getchar();
        }
        if (!flag2) memset(fire, 0x3f, sizeof(fire));
        if (flag) {
            printf("1\n");  
            continue;
        }
        BFS_F();
        int B = BFS();
        if (!B) {
            printf("IMPOSSIBLE\n"); 
        } else printf("%d\n", B + 1);
    }
    return 0;
}