[hdu]Tempter of the Bone

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 85139    Accepted Submission(s): 23194

Problem Description

The doggie found a bone in an ancient maze, which fascinated him a lot. However, when he picked it up, the maze began to shake, and the doggie could feel the ground sinking. He realized that the bone was a trap, and he tried desperately to get out of this maze.

The maze was a rectangle with sizes N by M. There was a door in the maze. At the beginning, the door was closed and it would open at the T-th second for a short period of time (less than 1 second). Therefore the doggie had to arrive at the door on exactly the T-th second. In every second, he could move one block to one of the upper, lower, left and right neighboring blocks. Once he entered a block, the ground of this block would start to sink and disappear in the next second. He could not stay at one block for more than one second, nor could he move into a visited block. Can the poor doggie survive? Please help him.

 

Input

The input consists of multiple test cases. The first line of each test case contains three integers N, M, and T (1 < N, M < 7; 0 < T < 50), which denote the sizes of the maze and the time at which the door will open, respectively. The next N lines give the maze layout, with each line containing M characters. A character is one of the following:

'X': a block of wall, which the doggie cannot enter; 
'S': the start point of the doggie; 
'D': the Door; or
'.': an empty block.

The input is terminated with three 0's. This test case is not to be processed.

 

Output

For each test case, print in one line "YES" if the doggie can survive, or "NO" otherwise.

 

Sample Input

4 4 5
S.X.
..X.
..XD
....
3 4 5
S.X.
..X.
...D
0 0 0

 

Sample Output

NO
YES

 

题意：小狗走迷宫。S是开始D是结束，X是墙，问能否在T时刻恰好到达重点。注意是恰好。

题解：因为问的是恰好，所以应该用的是深度优先搜索。注意奇偶剪枝，回溯法也是个重点。

#include "stdio.h"
#include "cmath"
#include "iostream"
using namespace std;
	int N,M,T;
	char a[10][10];
	int vis[10][10];
	int ax,ay,bx,by,flag;
	int dir[4][2]={{0,-1},{0,1},{-1,0},{1,0}};//上下左右 

void dfs(int x,int y,int t)
{
	int n,m,i;
	if(x==bx&&y==by&&t==T)
	{
		flag=1;return ;
	}
	if(t>=T)return ;
	for(i=0;i<4;i++)//遍历上下左右 
	{
		if(x+dir[i][0]>=1&&x+dir[i][0]<=N&&y+dir[i][1]>=1&&y+dir[i][1]<=M)//判断是否出格 
		{
			n=x+dir[i][0];m=y+dir[i][1]; 
			if(a[n][m]!='X'&&vis[n][m]!=1)
			{
				vis[n][m]=1;
				dfs(n,m,t+1);//开始深搜 
				if(flag>0)return ;
				vis[n][m]=0;//回溯还原标记 
			}
		}
	}
}
int main()
{
	int t;
	while(scanf("%d %d %d",&N,&M,&T)==3&&(N+M+T))
	{
		for(int i=1;i<=N;i++)
		{	getchar();//谜之回车 。。。 
			for(int j=1;j<=M;j++)
			{
				cin>>a[i][j];
				if(a[i][j]=='S')
				{
					ax=i;ay=j;
				}
				
				if(a[i][j]=='D')
				{
					bx=i;by=j;
				}
			}
		}
		flag=0;t=0;
		memset(vis,0,sizeof(vis));
		if(abs(ax-bx)+abs(ay-by)>T||(ax+bx+ay+by)%2!=T%2)//普通剪枝和奇偶剪枝。。感觉有一些弱 
		{cout<<"NO"<<endl;continue;}
		vis[ax][ay]=1;
		dfs(ax,ay,t);
		if(flag==0)cout<<"NO"<<endl;
		else cout<<"YES"<<endl;
	}
	return 0;
}