我们考虑可并堆维护，从深到浅贪心选取。
用priority_queue启发式合并的话，是60pts:
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<queue>
#include<ctime>
#define MAXN 3000010
using namespace std;
inline int read()
{int x=0,f=1;char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=-1; ch=getchar();}while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+(ch^48); ch=getchar();}return x*f;
}
int n,m,t,tot;
int head[MAXN],id[MAXN],fa[MAXN],c[MAXN];
long long ans;
struct Edge{int nxt,to;}edge[MAXN<<1];
priority_queue<int,vector<int>,less<int> >q[MAXN];
inline void add(int from,int to)
{edge[++t].nxt=head[from],edge[t].to=to;head[from]=t;
}
inline void solve(int x,int pre)
{id[x]=++tot;q[tot].push(x-1);for(int i=head[x];i;i=edge[i].nxt){int v=edge[i].to;if(v==pre) continue;solve(v,x);if(q[id[x]].size()<q[id[v]].size()) swap(id[x],id[v]);while(!q[id[v]].empty()){int cur=q[id[v]].top();q[id[v]].pop();q[id[x]].push(cur);}}for(int i=1;i<=c[x];i++){ans+=q[id[x]].top();q[id[x]].pop();if(q[id[x]].empty()) break;}
}
int main()
{#ifndef ONLINE_JUDGEfreopen("ce.in","r",stdin);#endifn=read(),m=read();for(int i=2;i<=n;i++) {fa[i]=read();fa[i]++;add(fa[i],i),add(i,fa[i]);}for(int i=1;i<=m;i++){int x;x=read();x++;c[x]++;}solve(1,0);printf("%lld\n",ans);return 0;
}用并查集维护的话，可以AC：
#include<iostream>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<algorithm>
#define MAXN 3000010
int n,m,num;
int cnt[MAXN],fa[MAXN],f[MAXN];
long long ans=0;
inline int find(int x){return x==f[x]?x:f[x]=find(f[x]);}
int main()
{#ifndef ONLINE_JUDGEfreopen("ce.in","r",stdin);#endifscanf("%d%d",&n,&m);for(int i=2;i<=n;i++) scanf("%d",&fa[i]),fa[i]++;for(int i=1;i<=m;i++){int x;scanf("%d",&x),x++;cnt[x]++;}for(int i=1;i<=n;i++) f[i]=(cnt[i]?i:fa[i]);for(int i=n;i>=1;i--){int x=find(i);if(cnt[x]){ans+=i-1;cnt[x]--;if(cnt[x]==0) f[x]=find(fa[x]);}}printf("%lld\n",ans);return 0;
}
