Set

#include <bits/stdc++.h>
using namespace std;
//...
set<int> S={2 , 3 , 4 , 7 , 8 , 9};
int main(void){
    int X=7;
    auto a=S.upper_bound(X);
    cout<<*a<<endl;//first greater
    auto b=S.lower_bound(X);
    cout<<*b<<endl;//first greater/equal
    auto c=S.lower_bound(X);
    cout<<*--c<<endl;//first lower
    auto d=S.upper_bound(X);
    cout<<*--d<<endl;//first lower/equal
    return 0;
}