[UVa11624]Fire!（BFS）

题目链接：https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2671

题意：一个人在树林里要到出口，同时有几个火点在扩展。问这个人能不能跑出树林，最少多少步能跑出去。

先扩展人，再扩展火。在同一个队列里扩展，所以要记下接下来要扩展几个人或者火点。

#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <climits>
#include <complex>
#include <fstream>
#include <cassert>
#include <cstdio>
#include <bitset>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <ctime>
#include <set>
#include <map>
#include <cmath>

using namespace std;

typedef pair<int, int> PII;
typedef struct P {
    int x;
    int y;
    int s;
    P() {}
    P(int xx, int yy, int ss) : x(xx), y(yy), s(ss) {}
}P;

const int inf = 0x7f7f7f;
const int maxn  = 1010;
const int dx[4] = {0, 0, 1, -1};
const int dy[4] = {1, -1, 0, 0};

int r, c, jx, jy;
vector<PII> fir;
int ans;
int vis[maxn][maxn];
char G[maxn][maxn];

inline int min(int x, int y) {
    return x < y ? x : y;
}

bool judge(int x, int y) {
    if(!vis[x][y] && G[x][y] == '.' && x >= 0 && x < r && y >= 0 && y < c) {
        return 1;
    }
    return 0;
}

bool success(int x, int y) {
    if(G[x][y] == 'J' && 
      (x == 0 || x == r - 1 || y == 0 || y == c - 1)) {
        return 1;
    }
    return 0;
}

void init() {
    memset(vis, 0, sizeof(vis));
    memset(G, 0, sizeof(G));
    ans = inf; 
    fir.clear();
}

void bfs() {
    P q[maxn*maxn];
    int front = 0, tail = 0;
    int fcnt = 0, jcnt = 1, tmp = 0;
    for(int i = 0; i < fir.size(); i++) {
        q[tail++] = P(fir[i].first, fir[i].second, 0);
        vis[fir[i].first][fir[i].second] = 1;
        fcnt++;
    }
    q[tail++] = P(jx, jy, 0); vis[jx][jy] = 1;
    while(front < tail) {
        for(int k = 0; k < fcnt; k++) {
            P f = q[front++];
            for(int i = 0; i < 4; i++) {
                int xx = f.x + dx[i];
                int yy = f.y + dy[i];
                if(judge(xx, yy) && G[xx][yy] != 'F') {
                    vis[xx][yy] = 1;
                    if(G[xx][yy] != 'J') {
                        G[xx][yy] = 'F';
                    }
                    q[tail++] = P(xx, yy, f.s+1); tmp++;
                }
            }
        }
        fcnt = tmp; tmp = 0;
        for(int k = 0; k < jcnt; k++) {
            P j = q[front++];
            if(success(j.x, j.y)) {
                ans = min(ans, j.s);
            }
            for(int i = 0; i < 4 ; i++) {
                int xx = j.x + dx[i];
                int yy = j.y + dy[i];
                if(judge(xx, yy) && G[xx][yy] != 'F') {
                    vis[xx][yy] = 1;
                    if(G[xx][yy] != 'F') {
                        G[xx][yy] = 'J';
                    }
                    q[tail++] = P(xx, yy, j.s+1); tmp++;
                }
            }
        }
        jcnt = tmp; tmp = 0;
    }
}

int main() {
    int T;
    scanf("%d", &T);
    while(T--) {
        init();
        scanf("%d %d", &r, &c);
        for(int i = 0; i < r; i++) {
            scanf("%s", G[i]);
        }
        for(int i = 0; i < r; i++) {
            for(int j = 0; j < c; j++) {
                if(G[i][j] == 'J') {
                    jx = i; jy = j;
                }
                if(G[i][j] == 'F') {
                    fir.push_back(PII(i, j));
                }
            }
        }
        bfs();
        if(ans == inf) {
            printf("IMPOSSIBLE
");
        }
        else {
            printf("%d
", ans+1);
        }
    }
    return 0;
}