BZOJ2152

Posted 2020-10-15 Stump

http://www.lydsy.com/JudgeOnline/problem.php?id=2152

还是点分治棵题

分治写法一样

只是维护的信息是路径长度mod 3的余数

#include<cstdio>
#include<algorithm>
#define FOR(i,s,t) for(register int i=s;i<=t;++i)
#define ROF(i,s,t) for(register int i=s;i>=t;--i)
#define VIS(now) for(register int e=las[now];e;e=nxt[e])
using std::sort;
typedef long long ll;
ll ans,_ans,g;
int n,k,tot;
const int N=100011;
int sz[N],vis[N];
int dis[N],t[3];
int las[N],nxt[N<<1],w[N<<1],f[N],to[N<<1],son[N<<1];
inline int max(int a,int b){
	return a>b?a:b;
}
inline void add(int x,int y,int z){
	nxt[++tot]=las[x];las[x]=tot;w[tot]=z;to[tot]=y;
} 
inline int find(int rt){
	static int qn,que[N];
	que[qn=1]=rt,f[rt]=0;
	int v,u,mx=n,G=0;
	FOR(ql,1,qn){
		sz[u=que[ql]]=1,son[u]=0;
		VIS(u)
			if(!vis[v=to[e]]&&v!=f[u]){
				f[v]=u;
				que[++qn]=v;
			}
	}
	ROF(ql,qn,1){
		u=que[ql],v=f[u];
		if(qn-sz[u]>son[u])son[u]=qn-sz[u];
		if(son[u]<mx)G=u,mx=son[u];
		if(!v)break;
		sz[v]+=sz[u];
		if(sz[u]>son[v])son[v]=sz[u];
	}
	return G;
}
inline ll calc(int rt,int L){
	static int qn,que[N];
	int u,v;
	que[qn=1]=rt,dis[rt]=L,f[rt]=0;
	t[0]=t[1]=t[2]=0;
	FOR(ql,1,qn){
		++t[dis[u=que[ql]]%3];
		VIS(u)
			if(!vis[v=to[e]]&&v!=f[u])
				f[v]=u,dis[v]=dis[u]+w[e],que[++qn]=v;
	}
	return 1ll*t[0]*(t[0]-1)/2+1ll*t[1]*t[2];
}
inline void dfs(int x){
	int G=find(x);	
	vis[G]=1;
	ans+=calc(G,0);
	VIS(G)
		if(!vis[to[e]])
			ans-=calc(to[e],w[e]);
	VIS(G)
		if(!vis[to[e]])
			dfs(to[e]);
}
int x,y,z;
inline ll gcd(ll a,ll b){
	return b?gcd(b,a%b):a;
}
int main(){
	scanf("%d",&n);
	FOR(i,2,n){
		scanf("%d%d%d",&x,&y,&z);
		add(x,y,z);
		add(y,x,z);
	}
	dfs(1);
	ans=1ll*(ans<<1)+n;
	_ans=1ll*n*n;
	g=gcd(ans,_ans);
	printf("%lld/%lld\n",ans/g,_ans/g);
	return 0;
}