UVA11624：Fire!（BFS + 优化）

题意：J为人的位置，F（可能有多个）为火源，每1s火源都在往四周扩散，人每秒走一步，求最少走几步到地图边缘，若不能输出IMPOSSIBLE。

思路：先BFS求出火到达每个点的时间，再BFS求出人逃离的时间。

# include <stdio.h>
# include <string.h>
# include <queue>
using namespace std;
int n, m;
char map[1001][1001];
int fire[1001][1001], vis[1001][1001], inext[4][2] = {{1,0},{-1,0},{0,1},{0,-1}};
struct node
{
    int x, y, step;
};
queue<node>Q;
void setMan(int x, int y, int t)
{
    while(!Q.empty())
        Q.pop();
    int flag = 0;
    memset(vis, 0, sizeof(vis));
    node tmp;
    tmp.x = x;
    tmp.y = y;
    tmp.step = t;
    vis[x][y] = 1;
    Q.push(tmp);
    while(!Q.empty())
    {
        for(int i=0; i<4; ++i)
        {
            if(Q.front().x==0||Q.front().x==n-1 || Q.front().y==0 || Q.front().y==m-1)
            {
                flag = 1;
                break;
            }
            int mx = Q.front().x + inext[i][0];
            int my = Q.front().y + inext[i][1];
            if(mx<0||my<0||mx>=n||my>=m||vis[mx][my] || map[mx][my]=='#')
                continue;
            vis[mx][my] = 1;
            if(fire[mx][my] != -1 && Q.front().step+1 >= fire[mx][my])
                continue;
            tmp.x = mx;
            tmp.y = my;
            tmp.step = Q.front().step + 1;
            Q.push(tmp);
        }
        if(flag)
            break;
        Q.pop();
    }
    if(flag)
        printf("%d\n",Q.front().step+1);
    else
        puts("IMPOSSIBLE");
}
void setFire()
{
    node tmp;
    while(!Q.empty())
    {
        for(int i=0; i<4; ++i)
        {
            int mx = Q.front().x + inext[i][0];
            int my = Q.front().y + inext[i][1];
            if(mx<0 || my<0 || mx>=n || my>=m || map[mx][my]=='#')
                continue;
            if(fire[mx][my]!=-1)
                continue;
            fire[mx][my] = Q.front().step+1;
            tmp.x = mx;
            tmp.y = my;
            tmp.step = Q.front().step + 1;
            Q.push(tmp);
        }
        Q.pop();
    }

}
int main()
{
    int t, sx, sy;
    node tmp;
    scanf("%d",&t);
    while(t--)
    {
        while(!Q.empty())
            Q.pop();
        memset(fire, -1, sizeof(fire));
        memset(vis, 0, sizeof(vis));
        scanf("%d%d",&n,&m);
        for(int i=0; i<n; ++i)
            scanf("%s",map[i]);

        for(int i=0; i<n; ++i)
            for(int j=0; j<m; ++j)
            {
                if(map[i][j] == 'F')
                {
                    tmp.x = i;
                    tmp.y = j;
                    tmp.step = 0;
                    fire[i][j] = 0;
                    Q.push(tmp);
                }
                else if(map[i][j] == 'J')
                {
                    sx = i;
                    sy = j;
                }
            }
        setFire();
        setMan(sx, sy, 0);
    }
    return 0;
}