调了好久了，实在看不出了，求大佬指点

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
const int maxn =1e6+10;
int n;
struct edge{
	int to,val;
};
vector <edge> a[maxn];
int sta[maxn],stais[maxn],cnt=0,vis[maxn];//DFS1中所需的 
bool ring[maxn],root[maxn];//环和子树根 
int in1[maxn],in2[maxn];
int dep[maxn],dt[maxn];//子树深度（从下往上） 和经过当前点子树直径 
int dmax[maxn];//子树最大直径 
long long ans=0; //总答案 
void build (int from,int to,int val)
{
	edge x;
	x.to=to; x.val=val;
	a[from].push_back(x);
	return ;
}
void dfs1 (int u,int fa)
{
//	cout<<u<<' ';
	vis[u]=1,sta[++cnt]=u;
	stais[u]=1;
	for (int i=0;i<a[u].size();i++)
	{
		int v=a[u][i].to;
		
		if (v==fa) continue;
		if (!vis[v]) dfs1 (v,u);
		else if (stais[v])
		{
			for (int i=cnt;i>=1;i--)
			{
				ring[sta[i]]=1;
				if (sta[i]==v) break;
			}
		}
		
	}
	stais[u]=0;
	cnt-=1;
}
void dfs2 (int u,int fa)
{
	if (a[u].size()==1) {
		if (ring[a[u][0].to]){
			dmax[a[u][0].to]=dep[a[u][0].to]=a[u][0].val;
		}
		return ;
	}
	int dn=0;//子树深度当前最深的点的编号 
	bool flag=0;
	for (int i=0;i<a[u].size();i++)
	{
		int v=a[u][i].to;
		if (v==fa) continue;
		if (ring[v]) {
			flag=1;
			continue;
		}
		dfs2 (v,u);
		if (dep[v]+a[u][i].val>dep[u])
		{
			dep[u]=dep[v]+a[u][i].val;
			dt[u]=dep[u]+dep[a[u][dn].to]+a[u][dn].val;
			dn=i;
		}
		if (dmax[v]>dmax[u]) dmax[u]=dmax[v];
		if (dt[v]>dmax[u]) dmax[u]=dt[v];
	}
	if (dmax[u]>dmax[u]) dmax[u]=dmax[u];
	if (dt[u]>dmax[u]) dmax[u]=dt[u];
	if (flag) for (int i=0;i<a[u].size();i++){
		int v=a[u][i].to;
		if (ring[v]){
			dep[v]=dep[u];
			dmax[v]=dmax[u];
			break;
		}
	}
	return ;
}
int mtdt=0,max1=0,max2=0;
int has[maxn];
void dfs (int u,int fa,int dep)
{
	vis[u]=has[u]=1;
	mtdt=max(mtdt,dmax[u]);
	if (dmax[u]+dep>max1) max1=dmax[u]+dep;
	if (dmax[u]-dep>max2) max2=dmax[u]-dep;
	cout<<u<<" "<<dmax[u]<<" "<<dep<<endl;
	for (int i=0;i<a[u].size();i++)
	{
		int v=a[u][i].to;
		if (v==fa||!ring[v]||vis[v]) continue;
		dfs(v,u,dep+a[u][i].val);
	}
	
	return ;
}
void clearv (){
	for (int i=0;i<=n+1;i++)  vis[i]=0;
}
int main ()
{
	freopen ("all.in","r",stdin);
	scanf ("%d",&n);
	for (int i=1;i<=n;i++)
	{
		int x,y;
		scanf ("%d%d",&x,&y);
		in1[i]=x,in2[i]=y;
		if (in1[i]<i&&in1[x]==i)//处理二元环 
		{
			ans+=max(y,in2[x]);
			continue;
		}
		build (i,x,y);
		build (x,i,y);
	}
	for (int i=1;i<=n;i++)
	{
		if (vis[i]==0) dfs1 (i,0);
	}
	//cout<<endl; for (int i=1;i<=n;i++) cout<<ring[i]<<" ";
	for (int u=1;u<=n;u++)
	{
		if (ring[u])
		{
			for (int i=0;i<a[u].size();i++)
			{
				int v=a[u][i].to;
				if (ring[v]==0) //标记子树根节点 
				{
					root[v]=1;
					break;
				}
			}
		}
	}
	for (int t=1;t<=n;t++) 
	{
		if (root[t]) dfs2(t,0);
	}
	//for (int i=1;i<=n;i++) cout<<dep[i]<<" "<<dmax[i]<<endl;
	for (int t=1;t<=n;t++)
	{
		if (ring[t]&&!has[t])
		{
			int u=t;
			int t1=0,t2=0;
			for (int i=0;i<a[u].size();i++) {
				if (ring[a[u][i].to])
				{
					if (t1==0) t1=a[u][i].to;
					else{
						t2=a[u][i].to;
						break;
					}
				}
			}
			clearv();
			mtdt=0,max1=max2=dep[t];
			dfs (u,t1,0);
			int c=max(mtdt,max1+max2);
			clearv();
			mtdt=0,max1=max2=dep[t];
			dfs (u,t2,0);
			c=max(c,max1+max2);
			ans+=c;
		}
	}
	cout<<ans;
	return 0;
}