参考答案：

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    vector<int> v(n);

    for (int i = 0; i < n; ++i) cin >> v[i];
    sort(v.begin(), v.end());
    v.erase(unique(v.begin(), v.end()), v.end());

    cout << v.size() << '\n';
    for (int i = 0; i < v.size(); ++i) cout << v[i] << ' ';

    return 0;
}