That's not a standard header. It's an implementation detail of your compiler, and as such is not a reliable include from version to version, never mind for users of other compilers.
The correct headers here are
using namespace std;
This is a poor (and potentially dangerous) practice. The name
std is intentionally very short, so shouldn't be too much trouble to type.
typedef long long ll;
Why are we obfuscating the code like this? If we want an alias for this type, why give it such an unreadable name?
ll lower_bound(std::vector<ll> arr, ll tar, ll n)
This is nothing like the signature of
std::lower_bound. Even considering the version that doesn't take a comparator, the standard library version is much more flexible, accepting any matched pair of iterators, and returning an iterator.
I can guess what
arr is, but
n are mysterious, and there's not even a comment to help. I had to spend some time digging into the implementation to discover that
tar is the value we're searching for, and
n seems to be a signed version of
ll l = 0, h = n - 1;
One line per declaration, please. Declaring multiple variables (even worse - 1-char variables) is harder to read.
It appears that you're using an inclusive range for search. You'll find that half-open ranges are easier to work with.
ll mid = l + (h - l) / 2;
Well done for avoiding the common overflow bug here. Though I'd normally use
std::midpoint() to get the same result more clearly.
std::cin, so the second statement is redundant. And since we never use
stdio, there's nothing to be gained from untying it anyway.
std::cin >> n;
Never ignore input errors - we need to be checking whether
good() before we use
n. This isn't just a one-off; the whole code is fragile this way.
std::cout << "No " << n << std::endl;
Why are we flushing output after every line, with
std::endl? Is there a reason not to simply write an ordinary newline (
'\n')? If the writer won't produce our next input until it receives this output, then use a comment to say so.
Not necessary from
main() - reaching the end of function has the same effect.