template<class ForwardIterator, class T>
pair<ForwardIterator, ForwardIterator>
equal_range(ForwardIterator first,
ForwardIterator last, const T& value);
template<class ForwardIterator, class T, class Compare>
pair<ForwardIterator, ForwardIterator>
equal_range(ForwardIterator first,
ForwardIterator last, const T& value,
Compare comp);
Requires: The elements e of [first,last) shall be partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value shall imply !(value < e) or comp(e, value) shall imply !comp(value, e).
Returns:
make_pair(lower_bound(first, last, value),
upper_bound(first, last, value))
or
make_pair(lower_bound(first, last, value, comp),
upper_bound(first, last, value, comp))
Complexity: At most 2 * log2(last - first) + Ο(1) comparisons.