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Meteor Shower
Time Limit: 1000MS Memory Limit: 65536K
Total Submissions: 2467 Accepted: 676

Description

Bessie hears that an extraordinary meteor shower is coming; reports say that these meteors will crash into earth and destroy anything they hit. Anxious for her safety, she vows to find her way to a safe location (one that is never destroyed by a meteor) . She is currently grazing at the origin in the coordinate plane and wants to move to a new, safer location while avoiding being destroyed by meteors along her way.

The reports say that M meteors (1 ≤ M ≤ 50,000) will strike, with meteor i will striking point (Xi, Yi) (0 ≤ Xi ≤ 300; 0 ≤ Yi ≤ 300) at time Ti (0 ≤ Ti  ≤ 1,000). Each meteor destroys the point that it strikes and also the four rectilinearly adjacent lattice points.

Bessie leaves the origin at time 0 and can travel in the first quadrant and parallel to the axes at the rate of one distance unit per second to any of the (often 4) adjacent rectilinear points that are not yet destroyed by a meteor. She cannot be located on a point at any time greater than or equal to the time it is destroyed).

Determine the minimum time it takes Bessie to get to a safe place.

Input

* Line 1: A single integer: M
* Lines 2..M+1: Line i+1 contains three space-separated integers: Xi, Yi, and Ti

Output

* Line 1: The minimum time it takes Bessie to get to a safe place or -1 if it is impossible.

Sample Input

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

Sample Output

5

Source

USACO 2008 February Silver
这道题没什么,一道BFS而已,只是数据有点问题,看了Discuss才知道有超出300的数据我一生气就开了500,开始RE,后来一次AC,唯一值得纪念的就是用了STL的queue写了BFS,下次看来要用stack写DFS了的确很是方便,部分代码如下:
while(!q.empty()){
            point
=q.front();
            a
=point.x,b=point.y;
            
if(flag[a][b]==0)break;
            
for(int i=0;i<4;i++){
                
int x=a+dir[i][0];
                
int y=b+dir[i][1];
                
if(OK(x,y)&&(map[x][y]==0||map[x][y]>map[a][b]+1)){
                    
if(map[x][y])flag[x][y]=1;
                    map[x][y]
=map[a][b]+1;
                    point.x
=x,point.y=y;
                    q.push(point);    
                }

            }

            q.pop();        
        }

posted on 2009-03-30 09:50 KNIGHT 阅读(156) 评论(0)  编辑 收藏 引用

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