Answer 1

#include <algorithm>

This header file contains basic algorithms so that we don't have to write algorithms for finding maximum, minimum, sorting etc from scratch.
Some of the important algorithms in algorithms header are:
-> min
-> max
-> min_element
-> max_element
-> find
-> count
-> replace
-> reverse
-> sort
-> binary_search
->upper_bound
-> lower_bound
-> accumulate
-> set_union
-> set_intersection
-> set_difference
-> next_permutation
.. and many more

Refer "The C++ Standard library" by Nicolai Josuttis to know about STL containers, algorithms in detail