HDU 1072 Nightmare (BFS)

Nightmare

Time Limit : 2000/1000ms (Java/Other)   Memory Limit : 65536/32768K (Java/Other)

Total Submission(s) : 17   Accepted Submission(s) : 4

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Problem Description

Ignatius had a nightmare last night. He found himself in a labyrinth with a time bomb on him. The labyrinth has an exit, Ignatius should get out of the labyrinth before the bomb explodes. The initial exploding time of the bomb is set to 6 minutes. To prevent the bomb from exploding by shake, Ignatius had to move slowly, that is to move from one area to the nearest area(that is, if Ignatius stands on (x,y) now, he could only on (x+1,y), (x-1,y), (x,y+1), or (x,y-1) in the next minute) takes him 1 minute. Some area in the labyrinth contains a Bomb-Reset-Equipment. They could reset the exploding time to 6 minutes.

Given the layout of the labyrinth and Ignatius' start position, please tell Ignatius whether he could get out of the labyrinth, if he could, output the minimum time that he has to use to find the exit of the labyrinth, else output -1.

Here are some rules:
1. We can assume the labyrinth is a 2 array.
2. Each minute, Ignatius could only get to one of the nearest area, and he should not walk out of the border, of course he could not walk on a wall, too.
3. If Ignatius get to the exit when the exploding time turns to 0, he can't get out of the labyrinth.
4. If Ignatius get to the area which contains Bomb-Rest-Equipment when the exploding time turns to 0, he can't use the equipment to reset the bomb.
5. A Bomb-Reset-Equipment can be used as many times as you wish, if it is needed, Ignatius can get to any areas in the labyrinth as many times as you wish.
6. The time to reset the exploding time can be ignore, in other words, if Ignatius get to an area which contain Bomb-Rest-Equipment, and the exploding time is larger than 0, the exploding time would be reset to 6.

Input

The input contains several test cases. The first line of the input is a single integer T which is the number of test cases. T test cases follow.
Each test case starts with two integers N and M(1<=N,Mm=8) which indicate the size of the labyrinth. Then N lines follow, each line contains M integers. The array indicates the layout of the labyrinth.
There are five integers which indicate the different type of area in the labyrinth:
0: The area is a wall, Ignatius should not walk on it.
1: The area contains nothing, Ignatius can walk on it.
2: Ignatius' start position, Ignatius starts his escape from this position.
3: The exit of the labyrinth, Ignatius' target position.
4: The area contains a Bomb-Reset-Equipment, Ignatius can delay the exploding time by walking to these areas.

Output

For each test case, if Ignatius can get out of the labyrinth, you should output the minimum time he needs, else you should just output -1.

Sample Input

3
3 3
2 1 1
1 1 0
1 1 3
4 8
2 1 1 0 1 1 1 0
1 0 4 1 1 0 4 1
1 0 0 0 0 0 0 1
1 1 1 4 1 1 1 3
5 8
1 2 1 1 1 1 1 4 
1 0 0 0 1 0 0 1 
1 4 1 0 1 1 0 1 
1 0 0 0 0 3 0 1 
1 1 4 1 1 1 1 1 

Sample Output

4
-1
13


代码：0MS
首先要找最短路就要用的广搜。
只能保证代码的正确性，思路懂一点，但是换另一种方法做，就一直WA...
这道题和平时搜索题不同在于有炸弹时间重置点，所以并不是走过的点就不能再走，样例3就很好的说明了这一点。
这就有一个问题了：怎么判断下一个点能不能走，因为不能再用走过就标记的方法去判断了。
一些人是判断重置点只能去一次(明显，再去时间也只能将时间重置为6，而路径加长)。
但是我感觉还是存在在两个点之间跳，浪费时间的情况，所以我另外开了一个时间二维数组，如果在之后又要回到已经走过的点，
我就判断剩余时间是否比之前的大，广搜已经得出之前到的一定路径更短，要回到走过的点，一定要在之后可以走更远，否则回
到该点一定不是最优的情况。于是判断语句出来了：if(temp.time>T[temp.x][temp.y]) 就可以入队。
#include <iostream>
#include <stdio.h>
#include <queue>
#include <string.h>
using namespace std;
int map[20][20],Time[20][20];
int lx,ly,n,m;
int dis[4][2]={1,0,0,1,-1,0,0,-1};
struct node {
int x,y,time,run;                   //分别是：横座标，纵座标，剩余时间，走的步数。
};
int BFS(int fx,int fy,int time,int run)
{
    int i,j;
    node t,temp;
    t.x=fx;t.y=fy;t.time=time;t.run=run;   
    queue<node>Q;                               //起点入队。    Q.push(t);
    memset(Time,0,sizeof(Time));                //时间表清空。
    Time[t.x][t.y]=t.time;                      //起点的剩余时间当然是6.
    while(!Q.empty())
    {
      t=Q.front();
      Q.pop();
      if(t.time==0) continue;                  //因为时间一到0炸弹就炸了，就算刚好到终点或重置点也炸了。所以就不用再去处理了。
      else
      {
      if(map[t.x][t.y]==4) t.time=6;          //如果到4，时间重置为6.
      if(t.x==lx && t.y==ly)
      {
        return  t.run;                        // 到达终点返回步数，广搜最先到的一定是步数最少的。
      }

          for(i=0;i<4;i++)                   //向四个方向走。
          {
            temp.x=t.x+dis[i][0];
            temp.y=t.y+dis[i][1];
            temp.time=t.time-1;
            temp.run=t.run+1;
            if( temp.x>=0 && temp.x<n && temp.y>=0 && temp.y<m && map[temp.x][temp.y]!=0)  //判断是否可以走。
                   if(temp.time>Time[temp.x][temp.y])                                      //没走过，当然成立；如果走过，就像上面说的，判断是否会出现
              {                                                                            //更优解的可能。
                   Time[temp.x][temp.y]=temp.time;                                         //对时间表更新。理由同上。
                   Q.push(temp);
              }
          }
       }
      }
        return -1;
    }
int main()
{
    int t,i,j;
    int fx,fy;
    while(scanf("%d",&t)!=EOF)
    {
        while(t--)
        {
            memset(map,0,sizeof(map));
            scanf("%d%d",&n,&m);
            for(i=0;i<n;i++)
                for(j=0;j<m;j++)
            {
                scanf("%d",&map[i][j]);
                if(map[i][j]==2) {fx=i;fy=j;}                  //找起点。
                if(map[i][j]==3) {lx=i;ly=j;}                  //找终点。
            }
            i=BFS(fx,fy,6,0);                                 
            printf("%d\n",i);
        }
    }
    return 0;
}