【POJ1741】tree

题解：

点分治第一题
其实除了统计答案这一步，其余基本都是套路
代码同法二

//by sdfzchy#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>using namespace std;typedef long long LL;const int inf=(1<<30),N=10010;int n,k;inline int in(){    char ch=getchar();    int f=1,tmp=0;    while(ch<'0'||ch>'9') {if(ch=='-') f=-1;ch=getchar();}    while(ch>='0'&&ch<='9') {tmp=(tmp<<1)+(tmp<<3)+(ch-'0');ch=getchar();}    return tmp*f;}struct E{    int to,nxt,dis;}e[N<<1];int sum,root,ecnt;int d[N],dis[N],head[N];void add(int u,int v,int w){    ++ecnt; e[ecnt]=(E){v,head[u],w}; head[u]=ecnt;    ++ecnt; e[ecnt]=(E){u,head[v],w}; head[v]=ecnt;}bool vis[N];int siz[N];void dfs_root(int u,int F){    d[u]=0; siz[u]=1;    for(int i=head[u];i;i=e[i].nxt)    {        int v=e[i].to;        if(vis[v]||v==F) continue;        dfs_root(v,u);        siz[u]+=siz[v];        d[u]=max(siz[v],d[u]);    }    d[u]=max(d[u],sum-siz[u]);    if(d[u]<d[root]) root=u; }int q[N];void dfs_dep(int u,int F){    q[++q[0]]=dis[u];    for(int i=head[u];i;i=e[i].nxt)    {        int v=e[i].to;        if(vis[v]||v==F) continue;        dis[v]=dis[u]+e[i].dis;        dfs_dep(v,u);    }}int cal(int u,int cur){    int ret=0;    dis[u]=cur;    q[0]=0; dfs_dep(u,0);    sort(q+1,q+q[0]+1);    int l=1,r=q[0];    while(l<r)    {        if(q[l]+q[r]<=k) ret+=r-l,l++;        else r--;    }    return ret;}int ans;void dfs_sol(int u){    vis[u]=1;    ans+=cal(u,0);    for(int i=head[u];i;i=e[i].nxt)    {        int v=e[i].to;        if(vis[v]) continue;        ans-=cal(v,e[i].dis);        sum=siz[v]; root=0;        dfs_root(v,u);        dfs_sol(root);    }}int main(){    while(1)    {        ecnt=0; memset(head,0,sizeof(head));        memset(vis,0,sizeof(vis));        n=in(),k=in();        if(n==0) break;        for(int i=1,u,v,w;i<n;i++)        {            u=in(),v=in(),w=in();            add(u,v,w);        }        sum=n; root=0; ans=0;        d[0]=inf;        dfs_root(1,0);        dfs_sol(root);        printf("%d\n",ans);    }    return 0;}