https://pintia.cn/problem-sets/994805342720868352/problems/994805345078067200
就是建树，然后遍历。
#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
int a[N],b[N],n;
unordered_map<int,int>mp,l,r;
int build(int l1,int r1,int l2,int r2)
{
int root=a[l1];
int k=mp[root];
if(k>l2) l[root]=build(l1+1,l1+k-l2,l2,k-1);
if(k<r2) r[root]=build(l1+k-l2+1,r1,k+1,r2);
return root;
}
vector<int>ans;
void dfs(int u)
{
if(l[u]) dfs(l[u]);
if(r[u]) dfs(r[u]);
ans.push_back(u);
}
int main(void)
{
cin>>n;
for(int i=1;i<=n;i++) cin>>a[i];
for(int i=1;i<=n;i++) cin>>b[i],mp[b[i]]=i;
build(1,n,1,n);
dfs(a[1]);
cout<<ans[0];
return 0;
}