UVA 11624 搜索

给出1000*1000矩阵，含起点‘J’，路‘.‘，墙‘#’，火‘F‘；

火每秒蔓延一格，人每秒走一步

问人是否可以安全走出矩阵，不能被火碰到

先对所有火BFS一遍，记录每个点被烧到的时间

然后对人BFS一遍，若到每点前没被火烧即可走

#include "stdio.h"
#include "string.h"
#include "queue"
using namespace std;

const int dir[4][2]={ {1,0},{-1,0},{0,1},{0,-1} };
char str[1100][1100];
int n,m;
int fire[1100][1100],used[1100][1100];
struct node
{
    int x,y,t;
};

int judge_fire(int x,int y)
{
    if (x<0 || y<0 || x>=n || y>=m) return 0;
    if (used[x][y]==1) return 0;
    if (str[x][y]=='#') return 0;
    return 1;
}

void bfs_fire()
{
    queue<node>q;
    node cur,next;
    int i,j;
    memset(fire,-1,sizeof(fire));
    memset(used,0,sizeof(used));
    for (i=0;i<n;i++)
        for (j=0;j<m;j++)
        if (str[i][j]=='F')
        {
            cur.x=i;
            cur.y=j;
            cur.t=0;
            q.push(cur);
            used[i][j]=1;
            fire[i][j]=0;
        }
    while (!q.empty())
    {
        cur=q.front();
        q.pop();
        for (i=0;i<4;i++)
        {
            next.x=cur.x+dir[i][0];
            next.y=cur.y+dir[i][1];
            if (judge_fire(next.x,next.y)==0) continue;
            next.t=cur.t+1;
            used[next.x][next.y]=1;
            fire[next.x][next.y]=next.t;
            q.push(next);
        }
    }
}

int judge_people(node b)
{
    if (str[b.x][b.y]=='#') return 0;
    if (used[b.x][b.y]==1) return 0;
    if (b.t>=fire[b.x][b.y] && fire[b.x][b.y]!=-1) return 0;
    return 1;
}

int bfs_people()
{
    queue<node>q;
    node cur,next;
    int i,j;
    memset(used,0,sizeof(used));
    for (i=0;i<n;i++)
    {
        for (j=0;j<m;j++)
        if(str[i][j]=='J')
        {
            cur.x=i;
            cur.y=j;
            cur.t=0;
            q.push(cur);
            used[i][j]=1;
            break;
        }
        if (j!=m) break;
    }

    while (!q.empty())
    {
        cur=q.front();
        q.pop();
        for (i=0;i<4;i++)
        {
            next.x=cur.x+dir[i][0];
            next.y=cur.y+dir[i][1];
            next.t=cur.t+1;
            if (next.x<0 || next.y<0 || next.x>=n || next.y>=m) return next.t;
            if (judge_people(next)==0) continue;
            q.push(next);
            used[next.x][next.y]=1;
        }
    }
    return -1;
}
int main()
{
    int Case,i,ans;
    scanf("%d",&Case);
    while (Case--)
    {
        scanf("%d%d",&n,&m);
        for (i=0;i<n;i++)
            scanf("%s",str[i]);

        bfs_fire();
        ans=bfs_people();
        if (ans==-1)
            printf("IMPOSSIBLE\n");
        else
            printf("%d\n",ans);
    }
    return 0;
}