POJ 1741 树分治

思路：
http://blog.sina.com.cn/s/blog_6d5aa19a0100o73m.html

//By SiriusRen#include <cstdio>#include <cstring>#include <algorithm>using namespace std;#define N 20005int n,k,xx,yy,zz,first[N],next[N],v[N],w[N],tot;int deep[N],ans,root,size[N],f[N],vis[N],sum,d[N];void add(int x,int y,int z){w[tot]=z,v[tot]=y,next[tot]=first[x],first[x]=tot++;}void getroot(int x,int fa){    size[x]=1,f[x]=0;    for(int i=first[x];~i;i=next[i])if(!vis[v[i]]&&v[i]!=fa){        getroot(v[i],x);        size[x]+=size[v[i]];        f[x]=max(f[x],size[v[i]]);    }    f[x]=max(f[x],sum-f[x]);    if(f[x]<f[root])root=x;}void getdeep(int x,int fa){    deep[++deep[0]]=d[x];    for(int i=first[x];~i;i=next[i])        if(!vis[v[i]]&&v[i]!=fa&&d[x]+w[i]<=k)            d[v[i]]=d[x]+w[i],getdeep(v[i],x);}int calc(int x,int now){    d[x]=now,deep[0]=0,getdeep(x,0);    sort(deep+1,deep+deep[0]+1);    int t,l,r;    for(t=0,l=1,r=deep[0];l<r;)        if(deep[l]+deep[r]<=k)t+=r-l,l++;        else r--;    return t;}void work(int x){    ans+=calc(x,0),vis[x]=1;    for(int i=first[x];~i;i=next[i])if(!vis[v[i]]){        ans-=calc(v[i],w[i]),sum=size[v[i]];        root=0,getroot(v[i],root),work(root);    }}int main(){    while(scanf("%d%d",&n,&k)&&(n||k)){        memset(first,-1,sizeof(first));        memset(vis,0,sizeof(vis));        f[0]=10005,root=ans=tot=0,sum=n;        for(int i=1;i<n;i++){            scanf("%d%d%d",&xx,&yy,&zz);            add(xx,yy,zz),add(yy,xx,zz);        }        getroot(1,0),work(root);        printf("%d\n",ans);    }}