树的直径

即树的最长路径

#include<cstdio>
#include<cstring>
#include<vector>
#include<iostream>
#include<queue> 
#define MAXN 1000
using namespace std;
int vis[MAXN];//记录数的节点是否被查找过 
int dis[MAXN];//记录距离 
vector <pair <int ,int > > V[MAXN];//记录节点和他们的距离 
int n,m;
int ans;//记录最大距离 

int bfs(int d)
{
	memset(vis,0,sizeof(vis));
	memset(dis,0,sizeof(dis));
	queue <int >Q;
	Q.push(d);
	vis[d]=1;
	int point=0;
	while(!Q.empty())
	{
		int F=Q.front();
		Q.pop();
		if(dis[F]>ans){
			ans=dis[F];
			point=F;
		}
		pair<int ,int> t;
		for(int i=0 ; i<V[F].size() ; i++){
			t=V[F][i];
			if(dis[t.first]==0){
				t.first=1;
				dis[t.first]=dis[F]+t.second;
				Q.push(t.first);
			}
		}
	}
	return point;
} 

int main()
{
	int x,y,z;
	while(scanf("%d %d %d",&x,&y,&z)!=EOF){	
		V[x]. push_back ( make_pair (y,z));
		V[y]. push_back ( make_pair (x,z));
	}
	ans=0;
	int point=bfs(1);
	ans=0;
	bfs(point);
	printf("%d\n",ans);
	return 0;
}