hdu 3721 树的直径

思路：枚举+树的直径

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<cmath>
#define clr(x,y) memset(x,y,sizeof(x))
#define Maxn 3000
#define inf 100000000
using namespace std;
int head[Maxn],Max[Maxn],lMax[Maxn],id[Maxn],road[Maxn],e,n,ans,te;
bool vi[Maxn];
struct Edge{
    int u,v,next,val;
}edge[Maxn*2];
void init()
{
    clr(head,-1);
    clr(vi,0);
    clr(Max,0);
    clr(lMax,0);
    clr(road,0);
    clr(id,0);
    e=0;
}
void add(int u,int v,int val)
{
    edge[e].u=u,edge[e].v=v,edge[e].val=val,edge[e].next=head[u],head[u]=e++;
    edge[e].u=v,edge[e].v=u,edge[e].val=val,edge[e].next=head[v],head[v]=e++;
}
void dfs(int u,int col,int fa)
{
    int v,i;
    id[u]=col;
    for(i=head[u];i!=-1;i=edge[i].next){
        v=edge[i].v;
        if(v==fa) continue;
        dfs(v,col,u);
        if(Max[v]+edge[i].val>Max[u]){
            lMax[u]=Max[u];
            Max[u]=Max[v]+edge[i].val;
            road[u]=v;
        } else if(Max[v]+edge[i].val>lMax[u])
            lMax[u]=Max[v]+edge[i].val;
    }
    te=max(te,lMax[u]+Max[u]);
}
void predfs(int u,int d,int fa)
{
    int v,i;
    Max[u]=max(Max[u],d);
    for(i=head[u];i!=-1;i=edge[i].next){
        v=edge[i].v;
        if(v==fa) continue;
        if(road[u]==v) predfs(v,max(lMax[u],d)+edge[i].val,u);
        else predfs(v,max(Max[u],d)+edge[i].val,u);
    }
}
void solve()
{
    int i,j,u,v,val,a,b;
    ans=inf;
    for(i=0;i<e-1;i+=2){
        u=edge[i].u,v=edge[i].v,val=edge[i].val;
        clr(Max,0);
        clr(lMax,0);
        a=b=inf;
        te=0;
        dfs(u,0,v);
        predfs(u,0,v);
        dfs(v,1,u);
        predfs(v,0,u);
        for(j=1;j<=n;j++){
            //cout<<j<<" "<<id[j]<<" "<<Max[j]<<endl;
            if(id[j]) a=min(a,Max[j]);
            else b=min(b,Max[j]);
        }
        ans=min(ans,max(a+b+val,te));
        //cout<<a<<" "<<b<<" "<<val<<endl;
       // cout<<"******************"<<endl;
    }
    return ;
}
int main()
{
    int i,j,u,v,val,t,Ca=0;
    scanf("%d",&t);
    while(t--){
        init();
        scanf("%d",&n);
        for(i=1;i<n;i++){
            scanf("%d%d%d",&u,&v,&val);
            u++,v++;
            add(u,v,val);
        }
        solve();
        printf("Case %d: %d\n",++Ca,ans);
    }
    return 0;
}