Poj2631--Roads in the North(树的直径)

#include <queue>
#include <cstdio>
#include <cstring>
#define N 100000+10
#define M 100000+10
using namespace std;
struct Edge
{
    int from, to, val, next;
}edge[M];
int head[N], cnt;
int dist[N]; bool vis[N];
int node;
int ans;
void init()
{
    cnt=0;
    memset(head, -1, sizeof(head));
}
int dfs(int sx)
{
    queue<int> Q;
    memset(dist, 0, sizeof(dist));
    memset(vis, false, sizeof(vis));
    vis[sx]=true;
    Q.push(sx);
    node=sx; ans=0;
    while(!Q.empty())
    {
        int u=Q.front();
        Q.pop();
        for(int i=head[u]; i!=-1; i=edge[i].next)
        {
            Edge E=edge[i];
            if(!vis[E.to]&&dist[E.to]<dist[u]+E.val)
            {
                dist[E.to]=dist[u]+E.val;
                vis[E.to]=true;
                Q.push(E.to);
                if(dist[E.to]>ans)
                {
                    ans=dist[E.to];
                    node=E.to;
                }
            }
        } 
    }
}
void solve()
{
    dfs(1);
    dfs(node);
    printf("%d\n", ans);
}
void add(int u, int v, int w)
{
    Edge E={u, v, w, head[u]};
    edge[cnt]=E;
    head[u]=cnt++;
}
int main()
{
    int a, b, c;
    init();
    while(scanf("%d%d%d", &a, &b, &c) != EOF)
    {
        add(a, b, c); 
        add(b, a, c);
    }
    solve();
    return 0;    
}