Efficient way to count occurrences of a key in a sorted array

Question

This was asked in on-site Microsoft interview.

Count the number of occurrences of a given key in an array.

I answered linear search because the elements may be s

Answer 1

For once, I will propose an implementation in C++.

size_t count(std::vector const& vec, int key)
{
  auto p = std::equal_range(vec.begin(), vec.end(), key);
  return std::distance(p.first, p.second);
}

equal_range uses a binary search, the result is equivalent to:

std::make_pair(std::lower_bound(vec.begin(), vec.end(), key),
               std::upper_bound(vec.begin(), vec.end(), key));

but the implementation should makes it slightly faster, even though all are in O(log N) (in terms of number of comparison).