Rescue（广度优先搜索 BFS）

最新推荐文章于 2021-04-06 22:32:03 发布

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最新推荐文章于 2021-04-06 22:32:03 发布

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分类专栏： BFS

本文链接：https://blog.youkuaiyun.com/qq_41555192/article/details/81301796

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本文探讨了一种基于ACM竞赛的算法问题，目标是在一个由墙壁、道路和守卫构成的监狱矩阵中找到最短路径，以拯救被困的朋友。通过使用广度优先搜索(BFS)算法，并结合优先队列来优化搜索过程，文章详细解释了如何计算到达目标位置所需的最小时间。

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网址：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemId=649

Angel was caught by the MOLIGPY! He was put in prison by Moligpy. The prison is described as a N * M (N, M <= 200) matrix. There are WALLs, ROADs, and GUARDs in the prison.

Angel's friends want to save Angel. Their task is: approach Angel. We assume that "approach Angel" is to get to the position where Angel stays. When there's a guard in the grid, we must kill him (or her?) to move into the grid. We assume that we moving up, down, right, left takes us 1 unit time, and killing a guard takes 1 unit time, too. And we are strong enough to kill all the guards.

You have to calculate the minimal time to approach Angel. (We can move only UP, DOWN, LEFT and RIGHT, to the neighbor grid within bound, of course.)

Input

First line contains two integers stand for N and M.

Then N lines follows, every line has M characters. "." stands for road, "a" stands for Angel, and "r" stands for each of Angel's friend.

Process to the end of the file.

Output

For each test case, your program should output a single integer, standing for the minimal time needed. If such a number does no exist, you should output a line containing "Poor ANGEL has to stay in the prison all his life."

Sample Input

7 8
#.#####.
#.a#..r.
#..#x...
..#..#.#
#...##..
.#......
........

Sample Output

#include<iostream>
#include<bits/stdc++.h>
using namespace std;
const int M=200+10;
char str[M][M];
int vis[M][M];
int n,m;
struct node
{
	int x,y,step;
}e1,e2;

bool operator < (node a,node b){
	return a.step >b.step ;
}

void BFS(int x1,int y1){
	priority_queue<node>que;//优先队列 
	memset(vis,0,sizeof(vis));
	e1.x=x1,e1.y=y1,e1.step=0;//起始位置 
	que.push(e1);//入栈 
	vis[e1.x][e1.y]=1;//标记 
	int ans=-1; 
	while(!que.empty()){
	    e1=que.top();
	    que.pop();//清空栈
		if(str[e1.x][e1.y]=='r'){//判断是不是终点 
	     	ans=e1.step;
			break;
	    }
		int p[4][2]={1,0,-1,0,0,1,0,-1};
    	for(int i=0;i<4;i++){//走四次 ，四个方向 
		    e2.x=e1.x+p[i][0];
		    e2.y=e1.y+p[i][1];
		    if(str[e2.x][e2.y]=='#') continue;
		    if(vis[e2.x][e2.y]==1) continue;
		    if(e2.x<0||e2.x>=n||e2.y<0||e2.y>=m) continue;
		    if(str[e2.x][e2.y]=='x') e2.step=e1.step+2;//步数 
		    else e2.step=e1.step+1;
		    que.push(e2);//再次入栈 
		    vis[e2.x][e2.y]=1; //标记
		} 
	} 
	if(ans==-1) 
	cout<<"Poor ANGEL has to stay in the prison all his life."<<endl;
    else cout<<ans<<endl;
}
int main()
{
	int stx,sty;
	while(cin>>n>>m){
	    memset(str,0,sizeof(str));
	    for(int i=0;i<n;i++) scanf("%s",str[i]);//用字符串接受，如果不这样会超时 
	    for(int i=0;i<n;i++){
		    for(int j=0;j<m;j++){
			    if(str[i][j]=='a'){
				stx=i;sty=j;//起点 
			    }
		    }
	    }
	    BFS(stx,sty);
	}
 return 0;	
}