Grouping of poj 1741 Tree

In the solution paper, the path is divided into two types: After the root and without the root. Each time the root is processed, the remaining part of the sub-tree can be recursively processed, if you select the center of gravity of the tree each time, the maximum number of recursive logn layers can be ensured. After sorting the roots, the complexity of O (n) can be calculated, so the complexity of each layer nlogn, the maximum recursion of the logn layer, the total complexity of nlogn ^ 2

 #include <iostream>  #include <cstdio>  #include <cstring>  #include <algorithm>  using namespace std;  const int maxn=1e4+9;  int head[maxn],lon;  bool use[maxn];  int a[maxn],d[maxn],top,son[maxn];  int ans,n,m,mm;  struct  {      int next,to,w;  }e[maxn<<1];    void edgeini()  {      memset(head,-1,sizeof(head));      lon=-1;  }  void edgemake(int from,int to,int w)  {      e[++lon].to=to;      e[lon].w=w;      e[lon].next=head[from];      head[from]=lon;  }  int maxson[maxn];  int dp(int t,int from)  {      int now,tmp=1e5,ans;      son[t]=maxson[t]=0;      for(int k=head[t];k!=-1;k=e[k].next)      {          int u=e[k].to;          if(use[u]||u==from) continue;          now=dp(u,t);          if(maxson[now]<tmp)          {              tmp=maxson[now];              ans=now;          }          son[t]+=son[u];          maxson[t]=max(maxson[t],son[u]);      }      son[t]++;      maxson[t]=max(maxson[t],mm-son[t]);      if(maxson[t]<tmp) ans=t;      return ans;  }    void dfs(int t,int from)  {      a[++top]=d[t];      son[t]=0;      for(int k=head[t],u;k!=-1;k=e[k].next)      {          u=e[k].to;          if(use[u]||u==from) continue;          d[u]=d[t]+e[k].w;          dfs(u,t);          son[t]+=son[u];      }      son[t]++;  }    int cal(int l,int r)  {      int ret=0,now=r;      for(int i=l;i<=now;i++)      {          while(now>i&&a[i]+a[now]>m) now--;          ret+=now-i;      }      return ret;  }    void solve(int t)  {      int now=dp(t,0);      d[now]=top=0;      use[now]=1;      dfs(now,0);      int ret=0,tmp=2;      for(int k=head[now];k!=-1;k=e[k].next)      {          int u=e[k].to;          if(use[u]) continue;          sort(a+tmp,a+tmp+son[u]);          ret+=cal(tmp,tmp+son[u]-1);          tmp+=son[u];      }      sort(a+1,a+tmp);      ans+=cal(1,tmp-1)-ret;  //    printf("%d %d\n",now,ans);      for(int k=head[now];k!=-1;k=e[k].next)      {          int u=e[k].to;          if(use[u]||son[u]==1) continue;          mm=son[u];          solve(u);      }  }    int main()  {  //    freopen("in.txt","r",stdin);      while(scanf("%d %d",&n,&m),n&&m)      {          edgeini();          for(int i=1,from,to,w;i<n;i++)          {              scanf("%d %d %d",&from,&to,&w);              edgemake(from,to,w);              edgemake(to,from,w);          }          memset(use,0,sizeof(use));          ans=0;          mm=n;          solve(1);          cout<<ans<<endl;      }      return 0;  }