RT 代码：

#include<bits/stdc++.h>
using namespace std;
int m,bj=1,ans=-1,z[600][600],vis[600][600],r,l=1;
int dx[4]= {1,0,-1,0},dy[4]= {0,1,0,-1};
struct sb {
	int x,y,z;
};
sb a[2000000],b[2000000];
int cmp(sb c,sb d) {
	return c.z<d.z;
}
void push(int x,int y,int z) {
	a[++r].x=x;
	a[r].y=y;
	a[r].z=z;
	vis[x][y]=1;
}
void bfs() {
	push(0,0,0);
	while(l<=r) {
		int x2=a[l].x;
		int y2=a[l].y;
		int z2=a[l].z;
		while(z2>=b[bj].z-1) {
			vis[b[bj].x][b[bj].y]=1;
			vis[b[bj].x+1][b[bj].y]=1;
			if(b[bj].x-1>=0)vis[b[bj].x-1][b[bj].y]=1;
			vis[b[bj].x][b[bj].y+1]=1;
			if(b[bj].y-1>=0)vis[b[bj].x][b[bj].y-1]=1;
			bj++;
		}
		l++;
		if(z[x2][y2]==0) {
			ans=z2;
			return;
		}
		for(int i=0; i<4; i++) {
			int xx=x2+dx[i];
			int yy=y2+dy[i];
			if(xx<0||yy<0||xx>310||yy>310||vis[xx][yy])continue;
			push(xx,yy,z2+1);
		}
	}
}
int main() {
	cin>>m;
	for(int i=1; i<=m; i++) {
		cin>>b[i].x>>b[i].y>>b[i].z;
		z[b[i].x][b[i].y]=1;
		z[b[i].x+1][b[i].y]=1;
		z[b[i].x][b[i].y+1]=1;
		if(b[i].x-1>=0)z[b[i].x-1][b[i].y]=1;
		if(b[i].y-1>=0)	z[b[i].x][b[i].y-1]=1;
	}
	sort(b+1,b+1+m,cmp);
	bfs();
	cout<<ans;
	return 0;
}