P2895 [USACO08FEB] Meteor Shower S求助

#include<bits/stdc++.h> 
using namespace std; 
short int n; 
short int t[50001],x[50001],y[50001]; 
short int dx[4]={1,0,-1,0}; 
short int dy[4]={0,1,0,-1}; 
short int a[400][400]; 
struct node 
{ 
	short int x,y,t; short int pre;
}que[17000000];
int main() 
{
	cin>>n;
	memset(a,-1,305*305);
	for (int i=0; i<n; i++) 
	{
		cin>>x[i]>>y[i]>>t[i];
	
		if(a[x[i]][y[i]]==-1||a[x[i]][y[i]]>t[i])
			a[x[i]][y[i]]=t[i];
		if(x[i]-1>=0&&(a[x[i]-1][y[i]]==-1||a[x[i]-1][y[i]]>t[i]))
			a[x[i]-1][y[i]]=t[i];
		if(y[i]-1>=0&&(a[x[i]][y[i]-1]==-1||a[x[i]][y[i]-1]>t[i]))
			a[x[i]][y[i]-1]=t[i];
		if(a[x[i]+1][y[i]]==-1||a[x[i]+1][y[i]]>t[i])
			a[x[i]+1][y[i]]=t[i];
		if(a[x[i]][y[i]+1]==-1||a[x[i]][y[i]+1]>t[i])
			a[x[i]][y[i]+1]=t[i];
	}
	if (a[0][0]==-1) 
	{
		cout<<0;
		return 0;
	}
	int head,tail; 
	head=0;tail=1; 
	que[tail].x=0; 
	que[tail].y=0; 
	que[tail].t=0; 
	que[tail].pre=0; 
	while (head<tail) 
	{ 
		head++; 
		for (int i=0;i<4;i++) 
		{ 
			int xx=que[head].x+dx[i]; 
			int yy=que[head].y+dy[i]; 
			if (xx>=0 and yy>=0 and (que[head].t+1<a[xx][yy] or a[xx][yy]==-1))
			{ 
				tail++; 
				que[tail].x=xx; 
				que[tail].y=yy; 
				que[tail].t=que[head].t+1; 
				que[tail].pre=head; 
				if (a[que[tail].x][que[tail].y]==-1) 
				{
					cout<<que[tail].t; return 0; 
				} 
			} 
		} 
	} 
	cout<<-1; 
	return 0; 
}