set大法好啊~
#include <set>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main() {
int n;
cin >> n;
set<int> seen;
for (int i = 0; i < n; ++i) {
int x;
cin >> x;
seen.insert(x);
}
cout << seen.size() << endl;
for (auto &it : seen) {
cout << it << " ";
}
cout << endl;
return 0;
}