HDOJ1010 (DFS+奇偶剪枝)

Tempter of the Bone

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 107043    Accepted Submission(s): 29107

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

思路：奇偶剪枝，见代码。

注意：这题最好用cin输入。

#include <iostream>
#include <string.h>
#include <algorithm>
using namespace std;
const int MAXN=10;
int n,m,T;
char mz[MAXN][MAXN];
int vis[MAXN][MAXN];
int sy,sx,gy,gx;
int dy[4]={0,1,0,-1};
int dx[4]={1,0,-1,0};
bool dfs(int y,int x,int step)
{
    if(y==gy&&x==gx&&step==T)
    {
        return true;
    }
    int rem=T-step;
    int need=abs(y-gy)+abs(x-gx);
    if(rem<need||(rem-need)%2!=0)//奇偶剪枝:迂回,若能到达终点,那么至少所需的步数与还剩余的步数奇偶性相同 
    {
        return false;
    }
    for(int i=0;i<4;i++)
    {
        int ny=y+dy[i];
        int nx=x+dx[i];
        if(0<=ny&&ny<n&&0<=nx&&nx<m&&mz[ny][nx]!='X')
        {
            if(!vis[ny][nx])
            {
                vis[ny][nx]=1;
                if(dfs(ny,nx,step+1))
                {
                    return true;
                }
                vis[ny][nx]=0;
            }    
        }
    }
    return false;
}
int main()
{
    while(cin>>n>>m>>T&&(n+m+T)!=0)
    {
        memset(vis,0,sizeof(vis));
        for(int i=0;i<n;i++)
        {
            for(int j=0;j<m;j++)
            {
                cin>>mz[i][j];
                if(mz[i][j]=='S')
                {
                    sy=i;
                    sx=j;
                }
                else if(mz[i][j]=='D')
                {
                    gy=i;
                    gx=j;
                }
            }
        }
        vis[sy][sx]=1;
        if(dfs(sy,sx,0))
        {
            cout<<"YES"<<endl;
        }
        else
        {
            cout<<"NO"<<endl;    
        }
    }
    return 0;
}