题目来源：UVa 10308 Roads in the North

题意：求距离最远的2点之间的距离

思路：裸的树的直径 或者树形DP

#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int maxn = 100010;
struct node
{
	int to, w;
	node(){}
	node(int to, int w): to(to), w(w) {} 
	bool operator < (const node& rhs) const
	{
		return w < rhs.w;
	}
};
vector <node> G[maxn];
int dis[maxn];
bool vis[maxn];
int path[maxn];
int p, leaf, n;


void BFS(int s)
{
	memset(vis, 0, sizeof(vis));
	memset(dis, 0, sizeof(dis));
	memset(path, 0, sizeof(path));
	queue <int> Q;
	Q.push(s);
	vis[s] = true;
	while(!Q.empty())
	{
		int u = Q.front();
		Q.pop();
		for(int i = 0; i < G[u].size(); i++)
		{
			node x = G[u][i];
			int v = x.to;
			if(vis[v])
				continue;
			vis[v] = true;
			dis[v] = dis[u] + x.w;
			path[v] = u;
			//printf("%d\n", dis[v]);
			if(leaf < dis[v])
			{
				p = v;
				leaf = dis[v];
			}
			Q.push(v);
		}
	}
}


int main()
{
	char str[100];
	while(gets(str))
	{
		for(int i = 1; i <= 10000; i++)
			G[i].clear();
		if(strlen(str) == 0)
		{
			puts("0");
			continue;
		}
		int u, v, w;
		sscanf(str, "%d %d %d", &u, &v, &w);
		G[u].push_back(node(v, w));
		G[v].push_back(node(u, w));
		while(gets(str) && strlen(str) > 0)
		{
			sscanf(str, "%d %d %d", &u, &v, &w);
			G[u].push_back(node(v, w));
			G[v].push_back(node(u, w));
		}
		leaf = 0;
		BFS(1);
		leaf = 0;
		BFS(p);
		printf("%d\n", leaf);
	}
	return 0;
}
/*
5 1 6
1 4 5
6 3 9
2 6 8
6 1 7
*/