pat-1021* Deepest Root

判断图是否都连接构成树，求使树高最大的根

实际上求图上两点间最大距离

先用并查集判断共有几个部分

bfs求距离
先任取一点开始bfs，得到最远的叶节点
以此叶节点再bfs可得

#include<iostream>
using namespace std;
#include<string>
#include<vector>
#include<queue>
#include<set>
#include<memory.h>
#include<algorithm>
#define MAXV 10001

int dist[MAXV];  // store distance
int points[MAXV];  // store root
vector<int> adj_list[MAXV];  //store neighbor

//return the  max distance
int bfs(int start , int V)
{
	memset(dist,-1,sizeof(dist));
	dist[start] = 0;
	queue<int> q;
	q.push(start);
	int dmax = 0;
	while(!q.empty())
	{
		int now = q.front();
		q.pop();
		int d = dist[now];
		for(int i = 0; i < adj_list[now].size(); i++)
		{
			//not visit before
			if(dist[ adj_list[now][i] ] == -1)
			{
				dist[ adj_list[now][i] ] = d +1;
				q.push(adj_list[now][i]);
			}
		}

		if(d > dmax)
		{
			dmax = d;
		}
	}
	return dmax;
}


int find(int pos)
{
	if(points[pos] == -1)
		return pos;
	return points[pos] = find(points[pos]);
}

int merge(int a,int b)
{
	int roota = find(a);
	int rootb = find(b);
	if(roota == rootb)
		return 0;

	points[roota] = rootb;
	return 1;

}

int main()
{
	int N;
	int a;
	int b;

	cin>>N;
	memset(points,-1,sizeof(points));

	for(int i=0;i<N-1;i++)
	{
		cin>>a;
		cin>>b; 
		merge(a,b);
		adj_list[a].push_back(b);
		adj_list[b].push_back(a);
	}

	//count components
	int cnt = 0;
	for(int i=1;i<=N;i++)
	{
		if(points[i]==-1)
			cnt++;
	}

	set<int> first;
	set<int> total;

	if(cnt != 1)
	{
		cout<<"Error: "<<cnt<<" components"<<endl;
		return 0;
	}
	else
	{
		int dmax = bfs(1,N);
		for(int i = 1 ;i <= N;i++ )
		{
			if(dist[i] == dmax)
			{
				first.insert(i);
				total.insert(i);
			}
		}
		dmax = bfs(*first.begin(),N);
		for(int i = 1 ;i <= N;i++ )
		{
			if(dist[i] == dmax)
			{
				first.insert(i);
				total.insert(i);
			}
		}

		for( set<int>::iterator it = total.begin() ; it != total.end() ; ++it )        
		{            
			cout << *it << endl;        
		}

	}



}