BZOJ 2599 Race(树分治)

题目链接：http://61.187.179.132/JudgeOnline/problem.php?id=2599

题意：给一棵树,每条边有权.求一条路径,权值和等于K,且边的数量最小.

题意：每次找到当前树的重心作为树根，查找通过当前树根的路径。

 #include<algorithm>
 #include<cstdio>
 #include<cmath>
 #include<cstring>
 #include<iostream>
 #include<ctime>
 int tot,go[],next[],first[],val[];
 int size[],F[],cnt[],a[],root,n,K,vis[];
 int dis[],ans,sz,h[],sum;
 long long sx;
 int read(){
     char ch=getchar();int t=,f=;
     while (ch<''||ch>'') {
         if (ch=='-') f=-;
         ch=getchar();
     }
     while (''<=ch&&ch<=''){
         t=t*+ch-'';
         ch=getchar();
     }
     return t*f;
 }
 void insert(int x,int y,int z){
     tot++;
     go[tot]=y;
     next[tot]=first[x];
     first[x]=tot;
     val[tot]=z;
 }
 void add(int x,int y,int z){
     insert(x,y,z);
     insert(y,x,z);
 }
 void findroot(int x,int fa){
     size[x]=;F[x]=;
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (pur==fa||vis[pur]) continue;
         findroot(pur,x);
         size[x]+=size[pur];
         F[x]=std::max(F[x],size[pur]);
     }
     F[x]=std::max(F[x],sum-size[x]);
     if (F[root]>F[x]) root=x;
 }
 void dfs1(int x,int fa){
     sx++;
     if (dis[x]>K) return;
     if (h[K-dis[x]]==sz) ans=std::min(ans,a[K-dis[x]]+cnt[x]);
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (pur==fa||vis[pur]) continue;
         cnt[pur]=cnt[x]+;
         dis[pur]=dis[x]+val[i];
         dfs1(pur,x);
     }
 }
 void dfs2(int x,int fa){
     if (dis[x]>K) return;
     if (h[dis[x]]!=sz) h[dis[x]]=sz,a[dis[x]]=cnt[x];
     else a[dis[x]]=std::min(a[dis[x]],cnt[x]);
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (pur==fa||vis[pur]) continue;
         dfs2(pur,x);
     }
 }
 int find(int x,int fa){
     int all=;
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (pur==fa||vis[pur]) continue;
         all+=find(pur,x);
     }
     return all;
 }
 void query(int x){
     h[]=++sz;
     a[]=;
     vis[x]=;
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (vis[pur]) continue;
         dis[pur]=val[i];
         cnt[pur]=;
         dfs1(pur,x);
         dfs2(pur,x);
     }
     for (int i=first[x];i;i=next[i]){
         int pur=go[i];
         if (vis[pur]) continue;
         root=;
         sum=find(pur,);
         findroot(pur,);
         query(root);
     }
 }
 int main(){
     n=read();K=read();
     for (int i=;i<n;i++){
         int x,y,z;
         x=read();
         y=read();
         z=read();
         x++;y++;
         add(x,y,z);
     }
     root=;
     F[]=n+;
     ans=n;
     sum=n;
     findroot(,);
     query(root);
     if (ans==n) ans=-;
     printf("%d\n",ans);
 }