1536. Ugly Numbers

typedef long long ll;
using namespace std;
struct cmp{
        bool operator()(const ll&a,const ll&b){
                return a>b;
        }
};
int main()
{
        set<ll>S;
        priority_queue<ll,vector<ll>,cmp>Q;
        S.insert(1);Q.push(1);
        for(int i=1;;i++){
                ll a=Q.top();Q.pop();
                if(i==1500) {cout<<"The 1500'th ugly number is "<<a<<"."<<endl;break;}
                if(S.find(a*2)==S.end()){
                        S.insert(a*2);
                        Q.push(a*2);
                }
                if(S.find(a*3)==S.end()){
                        S.insert(a*3);Q.push(a*3);
                }
                if(S.find(a*5)==S.end()){
                        S.insert(a*5);Q.push(a*5);
                }
        }

}