poj 2631 Roads in the North (树的直径)

 1#include <cstdio>
 2#include <cstring>
 3#include <iostream>
 4#include <algorithm>
 5#include <vector>
 6#include <queue>
 7#include <set>
 8#include <map>
 9#include <string>
10#include <cmath>
11#include <cstdlib>
12#include <ctime>
13#define fst first
14#define sec second
15#define lson l,m,rt<<1
16#define rson m+1,r,rt<<1|1
17#define ms(a,x) memset(a,x,sizeof(a))
18typedef long long LL;
19#define pi pair < int ,int >
20#define MP make_pair

 1using namespace std;
 2const double eps = 1E-8;
 3const int dx4[4]={1,0,0,-1};
 4const int dy4[4]={0,-1,1,0};
 5const int inf = 0x3f3f3f3f;
 6const int N=1E4+7;
 7int n,m;
 8vector < pi> edge[N];
 9int lst;
10int ans;
11int d[N];
12bool vis[N];

1void bfs( int s)
2{
3    ms(d,0x3f);
4    ms(vis,false);
5    queue<int>q;
6    q.push(s);
7    vis[s] = true;
8    d[s] =  0 ;

1    while (!q.empty())
2    {
3	int u = q.front() ; q.pop();
4	lst = u ;
5//	cout<<"u:"<<u<<endl;
6	int siz = edge[u].size();

 1	for ( int i = 0 ; i < siz ; i++)
 2	{
 3	    int v = edge[u][i].fst;
 4	    if (vis[v]) continue;
 5	    vis[v] = true;
 6	    d[v] = d[u] + edge[u][i].sec;
 7	    ans = max(d[v],ans);
 8	    q.push(v);
 9	}
10    }

 1    int mx = 0;
 2    for ( int i = 1 ; i <= n ; i++)
 3    {
 4//	cout<<"i:"<<i<<" d[i]:"<<d[i]<<endl;
 5	if (d[i]>mx)
 6	{
 7	    mx = d[i];
 8	    lst = i ;
 9	}
10    }

1}
2int main()
3{
4	#ifndef  ONLINE_JUDGE 
5	freopen("code/in.txt","r",stdin);
6  #endif

 1	int u,v,w;
 2	m = 0;
 3	n = 0;
 4	while (scanf("%d%d%d",&u,&v,&w)!=EOF)
 5	{
 6	    edge[u].push_back(make_pair(v,w));
 7	    edge[v].push_back(make_pair(u,w));
 8	    m++;
 9	    n = max(n,u);
10	    n = max(n,v);
11	}
12	ans = inf;
13	bfs(1);
14	ans = 0;
15//	cout<<"lst:"<<lst<<endl;
16	bfs(lst);
17	cout<<ans<<endl;

1  #ifndef ONLINE_JUDGE  
2  fclose(stdin);
3  #endif
4    return 0;
5}