[JZOJ] 5905. 黑暗之魂(darksoul)

基环树直径裸题

分别在子树做DP，环上做DP，环上可以用单调队列优化到\(O(n)\)

写起来很麻烦

#include<algorithm>
#include<iostream>
#include<cstdio>
#define int long long
using namespace std;

inline int rd(){
	int ret=0,f=1;char c;
	while(c=getchar(),!isdigit(c))f=c=='-'?-1:1;
	while(isdigit(c))ret=ret*10+c-'0',c=getchar();
	return ret*f;
}
#define pc putchar
#define space() pc(' ')
#define nextline() pc('\n')
void pot(int x){if(!x)return;pot(x/10);pc('0'+x%10);}
void out(int x){if(!x)pc('0');if(x<0)pc('-'),x=-x;pot(x);}

const int MOD = 1e9+7;

inline void upmax(int &x,int y){x=max(x,y);}
inline void upmin(int &x,int y){x=min(x,y);}

int add(int x,int y){return x+y>MOD?x+y-MOD:x+y;}
int sub(int x,int y){return x-y<0?x-y+MOD:x-y;}
int mul(int x,int y){return (long long)(1ll*x*y)%MOD;}
void Add(int &x,int y){x=add(x,y);}
void Sub(int &x,int y){x=sub(x,y);}
void Mul(int &x,int y){x=mul(x,y);}

const int MAXN = 1000005;
const int M = MAXN<<1;

int n;
int nex[M],to[M],wi[M];
int head[MAXN],ecnt=1;
inline void adde(int x,int y,int w){
	nex[++ecnt] = head[x];
	to[ecnt] = y;
	wi[ecnt] = w;
	head[x] = ecnt;
}
int cir[MAXN],vis[MAXN],lenc;

int dfn[MAXN],low[MAXN],tim;
int sta[MAXN],ins[MAXN],top;
void tarjan(int x,int frm){
	dfn[x]=low[x]=++tim;
	ins[sta[++top]=x]=1;
	for(int i=head[x];i;i=nex[i]){
		if(i==(frm^1)||i==frm)continue;
		int v=to[i];
		if(!dfn[v]){
			tarjan(v,i);
			upmin(low[x],low[v]);
		}else if(ins[v]){
			upmin(low[x],dfn[v]);
		}
	}
	if(dfn[x]!=low[x]) return;
	if(lenc>2) return;
	int elm;
	do{
		elm=sta[top--];
		ins[elm]=0;
		cir[++lenc]=elm;
	}while(elm!=x);
	if(lenc<=2) lenc=0;
}
int ans;
int f[MAXN],g[MAXN],h[MAXN];
void dfs(int x,int pre){
	for(int i=head[x];i;i=nex[i]){
		int v=to[i];
		if(v==pre||vis[v]) continue;
		dfs(v,x);
		upmax(ans,f[x]+f[v]+wi[i]);
		upmax(f[x],f[v]+wi[i]);
	}
}

int dis[MAXN];
void dfs2(int x){
	for(int i=head[cir[x]];i;i=nex[i]){
		int v=to[i];
		if(v!=cir[x+1])continue;
		dis[x]=wi[i];
		dfs2(x+1);
	}
}
void gfs(int x,int pre){
//	vis[x]=1;
	for(int i=head[x];i;i=nex[i]){
		int v=to[i];
		if(v==pre) continue;
		gfs(v,x);
		upmax(ans,f[x]+f[v]+wi[i]);
		upmax(f[x],f[v]+wi[i]);
	}
}

int q[MAXN*2],hd=1,tl;
signed main(){
	freopen("darksoul8.in","r",stdin);
	freopen("darksoul.out","w",stdout);
	n=rd();
	int x,y,w;
	for(int i=1;i<=n;i++){
		x=rd();y=rd();w=rd();
		if(x==y) continue;
		adde(x,y,w);adde(y,x,w);
	}
	for(int i=1;i<=n;i++) if(!dfn[i]) tarjan(i,0);
	for(int i=1;i<=lenc;i++) vis[cir[i]]=1;
	for(int i=1;i<=lenc;i++) dfs(cir[i],0);
	cir[lenc+1]=cir[1];
	dfs2(1);
	for(int i=1;i<=lenc;i++) g[i]=g[i+lenc]=f[cir[i]],dis[i+lenc]=dis[i];
	dis[0]=dis[lenc]; dis[2*lenc+1]=dis[1];
	int sumc=0;
	for(int i=1;i<=lenc;i++) sumc+=dis[i];
	for(int i=1;i<=lenc*2;i++) dis[i]+=dis[i-1];
	int j=1;
	while(dis[lenc]-dis[j-1]>sumc/2) j++;
	for(int k=j;k<=lenc;k++){
		while(hd<=tl&&g[q[tl]]-dis[q[tl]-1]<=g[k]-dis[k-1]) tl--;
		q[++tl]=k;
	}
	for(int i=lenc+1;i<=lenc*2;i++){
		while(hd<=tl&&dis[i-1]-dis[q[hd]-1]>sumc/2) hd++;
		h[i]=g[q[hd]]-dis[q[hd]-1]+g[i]+dis[i-1];
		while(hd<=tl&&g[q[tl]]-dis[q[tl]-1]<=g[i]-dis[i-1]) tl--;
		q[++tl]=i;
	}
	for(int i=lenc+1;i<=lenc*2;i++) upmax(ans,h[i]);
	out(ans+1);
	return 0;
}