点分治
经典点分治题。
每次求出重心,计算满足条件的点对(距离≤k),再减去多余的点对(不经过当前根的点对),然后递归处理其子树,累加即为答案。
代码:
#include<cstdio>
#include<cstring>
#include<algorithm>
#define MAXN 10000
using namespace std;
struct edge{
int next,to,dis;
}ed[MAXN*2+5];
int n,m,k,rt,ans,t,p;
int h[MAXN+5],sum[MAXN+5],dis[MAXN+5],ma[MAXN+5],dep[MAXN+5];
bool f[MAXN+5];
void addedge(int x,int y,int z){
ed[++k].next=h[x]; ed[k].to=y; ed[k].dis=z; h[x]=k;
}
void dfsrt(int x,int fa){//找重心
sum[x]=1; ma[x]=0;
for (int i=h[x];i;i=ed[i].next)
if (ed[i].to!=fa&&!f[ed[i].to]){
int v=ed[i].to; dfsrt(v,x);
sum[x]+=sum[v]; ma[x]=max(ma[x],sum[v]);
}
ma[x]=max(ma[x],t-sum[x]);
if (ma[x]<ma[rt]) rt=x;
}
void dfsdep(int x,int fa){//算距离
dep[++p]=dis[x];
for (int i=h[x];i;i=ed[i].next)
if (ed[i].to!=fa&&!f[ed[i].to]){
dis[ed[i].to]=dis[x]+ed[i].dis;
dfsdep(ed[i].to,x);
}
}
int dfssum(int x,int v){//求满足条件点对数
dis[x]=v; p=0; dfsdep(x,0);
sort(dep+1,dep+p+1);
int l=1,r=p,ret=0;
while (l<r)
if (dep[l]+dep[r]<=m) ret+=r-l,l++;
else r--;
return ret;
}
void dfsans(int x){//答案
f[x]=true; ans+=dfssum(x,0);//总方案
for (int i=h[x];i;i=ed[i].next)
if (!f[ed[i].to]){
int v=ed[i].to;
ans-=dfssum(v,ed[i].dis);//多余方案
rt=0; t=sum[v];
dfsrt(v,0); dfsans(rt);
}
}
int main(){
while(scanf("%d%d",&n,&m)&&(n||m)){
memset(f,false,sizeof(f));
memset(h,0,sizeof(h)); k=ans=0;
for (int i=1;i<n;i++){
int u,v,d; scanf("%d%d%d",&u,&v,&d);
addedge(u,v,d); addedge(v,u,d);
}
ma[(rt=0)]=0x7fffffff; t=n;
dfsrt(1,0);
dfsans(rt);
printf("%d\n",ans);
}
return 0;
}