用set去重吧，如果不要把个数排在前面直接用set而不是unordered_set了

#include<bits/stdc++.h>
using namespace std;

int main()
{
	int n;
	while(cin >> n){
		unordered_set<int>st;
		vector<int>ans;
		int sum = 0;
		
		for(int i = 0;i < n;i++){
			int num = 0;
			cin>>num;
			st.insert(num);
		}
		for(int i = 0;i < n;i++){
			int num = 0;
			cin>>num;
			if(st.count(num)){
				sum++;
				ans.push_back(num);
			}
		}
		cout<<sum<<endl;
		sort(ans.begin(), ans.end());
		for(auto &a : ans){
			cout<<a<<" ";
		}
		cout<<endl;
	}
 	return 0;
}