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;

}