ugly number （优先队列+集合）

#include <iostream>
#include<vector>
#include<queue>
#include<set>
using namespace std;
typedef long long LL;
const int coeff[3] = { 2,3,5 };
int main()
{
    priority_queue<LL, vector<LL>, greater<LL> >pq;
    set<LL>s;
    pq.push(1);
    s.insert(1);
    for (int i = 1;; i++)
    {
        LL x = pq.top();
        pq.pop();
        if (i == 1500)
        {
            cout << "The 1500'th ugly number is " << x <<"."<<endl;
            break;
        }
        for (int j = 0; j < 3; j++)
        {
            LL x2 = x*coeff[j];
            if (!s.count(x2))
            {
                s.insert(x2);
                pq.push(x2);
            }
        }
    }
}