std::lower_bound 和 std::upper_bound [on hold] 的时间复杂度

Question

I use std::upper_bound on a sorted vector, and it's clearly not working in logarithmic time, as I compare it with my own implementation of upper bound search. Why is it happening? My code:

for (size_t i = 1; i < sorted_vector.size(); ++i) {
    size_t m = std::upper_bound(
        sorted_vector.begin(),
        sorted_vector.begin() + i,
        values_to_search[i]) - sorted_vector.begin();

Total time here supposed to be O(n * log(n)) . When I use my own implementation of upper_bound on an array of size 50000 it takes about 0.05 seconds, while with std it's around 3 seconds.

Answer 1

Likely, your sorted_vector 's iterator is not a RandomAccessIterator like with std::vector .

Instead you may be dealing with a forward or bidirectional iterator, which would mean O(N) for “jumping” distances across the container in binary search rather than O(1) (which you've indicated you're expecting).

Check to make sure sorted_vector is a std::vector or similar!