Twice linear

• Time : $O(nlogn)$
• Space : $O(n)$

#include <bits/stdc++.h>
using namespace std;
class DoubleLinear
{
public:
    static int dblLinear(int n);
};
int DoubleLinear::dblLinear(int n) {
    unordered_set<int> us;
    priority_queue<int, vector<int>, greater<int>> q;
    q.push(1);
    auto insert = [&](int x) {
        if(!us.count(x)) {
            us.insert(x);
            q.push(x);
        }
    };
    while(n--) {
        auto u = q.top(); q.pop();
        us.erase(u);
        insert(u * 2 + 1);
        insert(u * 3 + 1);
    }
    return q.top();
}