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[3][N],ans;

il int gi(){
  RG int x=0,q=1; RG char ch=getchar();
  while ((ch<'0' || ch>'9') && ch!='-') ch=getchar();
  if (ch=='-') q=-1,ch=getchar();
  while (ch>='0' && ch<='9') x=x*10+ch-'0',ch=getchar();
  return q*x;
}

struct graph{
  
  struct edge{ int nt,to; ll dis; }g[N<<1];
  
  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[3];

il void bfs(RG int op,RG int S){
  for (RG int i=1;i<=n;++i) vis[i]=0;
  RG int h=0,t=1; q[t]=S,vis[S]=1,dis[op][S]=0;
  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]=1,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(19260817),n=gi();
  for (RG int i=1,u,v;i<n;++i){
    u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
    G[0].insert(u,v,w),G[0].insert(v,u,w);
  }
  for (RG int i=1,u,v;i<n;++i){
    u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
    G[1].insert(u,v,w),G[1].insert(v,u,w);
  }
  for (RG int i=1,u,v;i<n;++i){
    u=gi(),v=gi(); RG ll w; scanf("%lld",&w);
    G[2].insert(u,v,w),G[2].insert(v,u,w);
  }
  for (RG int i=1;i<=n;++i) pos[i]=i;
  random_shuffle(pos+1,pos+n+1);
  for (RG int T=1,rt;T<=25;++T){
    rt=pos[T]; RG ll ret=0;
    while (1){
      bfs(0,rt),bfs(1,rt),bfs(2,rt);
      RG ll res=0; RG int id=0;
      for (RG int i=1;i<=n;++i)
    if (res<dis[0][i]+dis[1][i]+dis[2][i])
      res=dis[0][i]+dis[1][i]+dis[2][i],id=i;
      if (ret<res) ret=res,rt=id; else break;
    }
    ans=max(ans,ret);
  }
  cout<<ans; return 0;
}