[UVA11624] Fire!
2017年10月19日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;
}