UVA 11624 Fire！（预处理bfs）

 

https://vjudge.net/problem/UVA-11624

UVa真的好卡，，

先预处理‘F’，地图上标记好每个位置火最早出现的时间，之后正常的跑bfs就行了，还是挺水的题

做题前不要莽，磨刀不误砍柴工，仔细想想再下手

最后一道简单搜索了啊，去刷进阶版！

 

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<cmath>
#define ll long long
#define mod 1000000007
#define inf 0x3f3f3f3f
using namespace std;
int n, m;
char a[1005][1005];
int vis[1005][1005];
int fire[1005][1005];
int dir[4][2] = {0, 1, 0, -1, 1, 0, -1, 0};
struct node
{
    int x, y, t;
};
queue<node> q;
void bfs()
{
    node h, next;
    while(! q.empty())
    {
        h = q.front();
        q.pop();
        for(int i = 0; i < 4; i ++)
        {
            int u = h.x + dir[i][0];
            int v = h.y + dir[i][1];
            if(u >= 1 && u <= n && v >= 1 && v <= m && a[u][v] != '#' && ! vis[u][v])
            {

                vis[u][v] = 1;
                next.x = u, next.y = v;
                next.t = h.t + 1;
                q.push(next);
                fire[u][v] = min(next.t, fire[u][v]);

            }
        }
    }

}
int bfs1(int x, int y)
{
    queue<node> q;
    node h, next;
    h.x = x, h.y = y, h.t = 0;
    vis[x][y] = 0;
    q.push(h);
    while(! q.empty())
    {
        h = q.front();
        q.pop();
        for(int i = 0; i < 4; i ++)
        {
            int u = h.x + dir[i][0];
            int v = h.y + dir[i][1];
            if(u >= 1 && u <= n && v >= 1 && v <= m && a[u][v] != '#' && (fire[u][v] == inf || fire[u][v] > h.t + 1) && vis[u][v] == -1)
            {
                next.x = u, next.y = v;
                next.t = h.t + 1;
                q.push(next);
                vis[u][v] = next.t;
                if((u == 1) || (u == n) || (v == 1) || (v == m))
                {
                    return next.t + 1;
                }
            }
        }
    }
    return -1;
}
int main()
{
    int t;
    scanf("%d", &t);
    while(t --)
    {
        while(! q.empty()) q.pop();
        node h;
        scanf("%d%d", &n, &m);
        int jx, jy;
        for(int i = 1; i <= n; i ++)
        {
            scanf("%s", a[i] + 1);
        }
        for(int i = 1; i <= n; i ++)
            for(int j = 1; j <= m; j ++)
            {
                if(a[i][j] == 'F')
                {
                    h.x = i, h.y = j, h.t = 0;
                    q.push(h);
                    fire[i][j] = 0;
                    vis[i][j] = 1;
                }
                if(a[i][j] == 'J')
                {
                   jx = i;
                   jy = j;
                }
            }
        memset(vis, 0, sizeof(vis));
        memset(fire, 0x3f, sizeof(fire));
        bfs();
        memset(vis, -1, sizeof(vis));
        if(jx == 1 || jx == n || jy == 1 || jy == m)
        {
            printf("1\n");
            continue;
        }
        int ans = bfs1(jx, jy);
        if(ans == -1) printf("IMPOSSIBLE\n");
        else printf("%d\n", ans);
    }
    return 0;
}