bzoj5152 [Wc2018]通道

题目链接

正解：不会做。

写一个爬山算法就过官方数据了(逃

具体来说就是每次随机一个根，然后迭代找最长路的那个点。

多随机几次取$max$就行了。正解以后再补。。

 #include <bits/stdc++.h>
 #define il inline
 #define RG register
 #define ll long long
 #define N (100005)

 using namespace std;

 int pos[N],vis[N],q[N],n;
 ll dis[][N],ans;

 il int gi(){
   RG int x=,q=; RG char ch=getchar();
   while ((ch<'' || ch>'') && ch!='-') ch=getchar();
   if (ch=='-') q=-,ch=getchar();
   while (ch>='' && ch<='') x=x*+ch-'',ch=getchar();
   return q*x;
 }

 struct graph{

   struct edge{ int nt,to; ll dis; }g[N<<];

   int head[N],num;

   il void insert(RG int from,RG int to,RG ll dis){
     g[++num]=(edge){head[from],to,dis},head[from]=num; return;
   }

 }G[];

 il void bfs(RG int op,RG int S){
   for (RG int i=;i<=n;++i) vis[i]=;
   RG int h=,t=; q[t]=S,vis[S]=,dis[op][S]=;
   while (h<t){
     RG int x=q[++h],v;
     for (RG int i=G[op].head[x];i;i=G[op].g[i].nt){
       v=G[op].g[i].to; if (vis[v]) continue;
       vis[v]=,q[++t]=v,dis[op][v]=dis[op][x]+G[op].g[i].dis;
     }
   }
   return;
 }

 int main(){
 #ifndef ONLINE_JUDGE
   freopen("tunnel.in","r",stdin);
   freopen("tunnel.out","w",stdout);
 #endif
   srand(),n=gi();
   for (RG int i=,u,v;i<n;++i){
     u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
     G[].insert(u,v,w),G[].insert(v,u,w);
   }
   for (RG int i=,u,v;i<n;++i){
     u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
     G[].insert(u,v,w),G[].insert(v,u,w);
   }
   for (RG int i=,u,v;i<n;++i){
     u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
     G[].insert(u,v,w),G[].insert(v,u,w);
   }
   for (RG int i=;i<=n;++i) pos[i]=i;
   random_shuffle(pos+,pos+n+);
   for (RG int T=,rt;T<=;++T){
     rt=pos[T]; RG ll ret=;
     while (){
       bfs(,rt),bfs(,rt),bfs(,rt);
       RG ll res=; RG int id=;
       for (RG int i=;i<=n;++i)
     if (res<dis[][i]+dis[][i]+dis[][i])
       res=dis[][i]+dis[][i]+dis[][i],id=i;
       if (ret<res) ret=res,rt=id; else break;
     }
     ans=max(ans,ret);
   }
   cout<<ans; return ;
 }