The <algorithm> library provides functions for a variety of tasks (e.g., sorting, searching, transforming, and partitioning) on ranges of elements. It works elegantly with all STL containers.

Always! Never rewrite a sorting or searching algorithm from scratch unless specifically asked in an interview. STL algorithms are highly optimized.

• std::sort(): $O(N \log N)$
• std::binary_search(): $O(\log N)$ on sorted ranges.
• std::lower_bound() / std::upper_bound(): $O(\log N)$ on sorted ranges.

cpp Standard
#include <algorithm>
#include <vector>

std::vector<int> v = {4, 2, 5, 1, 3};
std::sort(v.begin(), v.end()); // {1, 2, 3, 4, 5}

auto it = std::lower_bound(v.begin(), v.end(), 3); // it points to 3

• Using binary search algorithms on unsorted ranges (leads to undefined behavior).
• Using std::sort on std::list (use list::sort() instead because list iterators are not random access).
• Forgetting that lower_bound returns an iterator, which needs to be dereferenced or checked against end().