POJ [P2631] Roads in the North

树的直径

树的直径求法：

1. 任取一点u，找到树上距u最远的点s
2. 找到树上距s点最远的点t，s->t的距离即为所求

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <queue>
using namespace std;
int init(){
	int rv=0,fh=1;
	char c=getchar();
	while(c<'0'||c>'9'){
		if(c=='-') fh=-1;
		c=getchar();
	}
	while(c>='0'&&c<='9'){
		rv=(rv<<1)+(rv<<3)+c-'0';
		c=getchar();
	}
	return fh*rv;
}
const int MAXN=30005;
int n,m,head[MAXN],nume,q,dis[MAXN];
struct edge{
	int to,nxt,dis;
}e[MAXN<<1];
void adde(int from,int to,int dis){
	e[++nume].to=to;
	e[nume].nxt=head[from];
	head[from]=nume;
	e[nume].dis=dis;
}
bool f[MAXN];
int bfs(int s){
	queue<int>q;
	q.push(s);
	dis[s]=0;
	memset(dis,0,sizeof(dis));
	memset(f,0,sizeof(f));
	f[s]=1;
	while(!q.empty()){
		int u=q.front();q.pop();
		for(int i=head[u];i;i=e[i].nxt){
			int v=e[i].to;
			if(!f[v]){
				q.push(v);
				f[v]=1;
				dis[v]=dis[u]+e[i].dis;
			}
		}
	}
}
int main(){
	freopen("in.txt","r",stdin);
	int u,v,di;
	while(~scanf("%d%d%d",&u,&v,&di)){
		n=max(n,u);n=max(n,v);
		adde(u,v,di);adde(v,u,di);
	}
	bfs(1);
	int t=0,ma=0;
	for(int i=1;i<=n;i++){
		if(ma<dis[i]){
			ma=dis[i];
			t=i;
		}
	}
	//cout<<t<<endl;
	bfs(t);
	ma=0;
	//for(int i=1;i<=n;i++) cout<<dis[i]<<endl;
	for(int i=1;i<=n;i++){
		if(ma<dis[i]){
			ma=dis[i];
		}
	}
	cout<<ma;
	fclose(stdin);
	return 0;
}