傳送門:bzoj2870

題解

邊分裸題
邊分治學習筆記-litble

程式碼

#include<bits/stdc++.h>
#define pb push_back
using namespace std;
typedef long long ll;
const int N=2e5+10,inf=0x7f7f7f7f;

int n,m,v[N],S,MN,rt,sz[N];
int head[N],to[N<<1],nxt[N<<1],w[N<<1],tot=1;
vector<int>g[N];
 
bool vs[N];ll ans;

char cp;
inline void rd(int &x)
{
	cp=getchar();x=0;int f=0;
	for(;!isdigit(cp);cp=getchar()) if(cp=='-') f=1;
	for(;isdigit(cp);cp=getchar()) x=x*10+(cp^48);
	if(f) x=-x;
}

struct G{
	int num;
	struct P{
		int len,v;
		bool operator <(const P&ky)const{return v>ky.v;}
 

	}a[N];
	void dfs(int x,int fr,int dis,int mn)
	{
		int i,j;a[++num]=(P){dis,mn};
        for(i=head[x];i;i=nxt[i]){
			j=to[i];if(vs[i>>1] || j==fr) continue;
			dfs(j,x,dis+w[i],min(mn,v[j]));
		}
	}
}A,B;

inline void lk(int u,int v,int vv)
{
	to[++tot]=v;nxt[tot]=head[u];head[u]=tot;w[tot]
 
=vv;
	to[++tot]=u;nxt[tot]=head[v];head[v]=tot;w[tot]=vv;
}

void dfs(int x,int fr)
{
	int i,j;
	for(i=head[x];i;i=nxt[i]){
		j=to[i];if(j==fr) continue;
		g[x].pb(j);dfs(j,x);
	}
}

void reb()
{
	int x,i,j,sz,a,b;
    for(x=1;x<=n;++x){
        sz=g[x].size();
		if(sz<3){
			for(i=0;i<sz;++i){j=g[x][i];lk(x,j,(j<=m));}
		}else{
            a=++n;b=++n;v[a]=v[b]=v[x];lk(x,a,0);lk(x,b,0);
			for(i=0;i<sz;++i){j=g[x][i];(i&1)?g[b].pb(j):g[a].pb(j);}
		}
	}
}

void fdrt(int x,int fr)
{
	sz[x]=1;int i,j,k;
	for(i=head[x];i;i=nxt[i]){
		j=to[i];if(vs[i>>1] || j==fr) continue;
		fdrt(j,x);sz[x]+=sz[j];
        k=max(sz[j],S-sz[j]);
		if(k<MN){MN=k;rt=i;}
	}
}

void sol(int le)
{
	int i,j,la,lb,mx=0,rv,du=to[le^1],dv=to[le];
	vs[le>>1]=true;
    A.num=B.num=0;A.dfs(du,0,0,v[du]);B.dfs(dv,0,0,v[dv]);
	la=A.num;lb=B.num;
	sort(A.a+1,A.a+la+1);sort(B.a+1,B.a+lb+1);
	for(i=j=1;i<=la;++i){
		rv=A.a[i].v;
		for(;j<=lb && B.a[j].v>=rv;++j) mx=max(mx,B.a[j].len);
		if(j>1) ans=max(ans,(ll)rv*(mx+A.a[i].len+w[le]+1));
	}
	mx=0;
	for(i=j=1;i<=lb;++i){
		rv=B.a[i].v;
		for(;j<=la && A.a[j].v>=rv;++j) mx=max(mx,A.a[j].len);
		if(j>1) ans=max(ans,(ll)rv*(mx+B.a[i].len+w[le]+1));
	}
    la=sz[dv];lb=S-sz[dv];
	S=la;MN=N;fdrt(dv,0);if(MN<N) sol(rt);
	S=lb;MN=N;fdrt(du,0);if(MN<N) sol(rt);
}

int main(){
    int i,x,y;rd(n);m=n;
	for(i=1;i<=n;++i) rd(v[i]);
    for(i=1;i<n;++i){rd(x);rd(y);lk(x,y,1);}
    dfs(1,0);memset(head,0,(n+1)<<2);tot=1;reb();
    S=n;MN=N;fdrt(1,0);if(MN<N) sol(rt);
	printf("%lld",ans);
	return 0;
}