nth_element

The `nth_element` algorithm is a function in the C++ Standard Template Library (STL) that is used to rearrange elements in a container such that the nth element is in its final sorted position, while all the other elements are unordered.

The syntax for using `nth_element` is:

nth_element(start_iterator, nth_element_iterator, end_iterator);

where `start_iterator` and `end_iterator` are the iterators defining the range of elements to be sorted, and `nth_element_iterator` is an iterator pointing to the nth element.

For example, consider the following code:

#include <iostream>
#include <algorithm>
#include <vector>

int main()
{
    std::vector<int> vec = { 5, 2, 9, 1, 7, 3, 8, 6, 4 };

    std::nth_element(vec.begin(), vec.begin() + 4, vec.end());

    std::cout << "The fifth element is " << vec[4] << std::endl;

    return 0;
}

In this example, `nth_element` is used to rearrange the elements in the `vec` vector such that the fifth element (i.e., `vec[4]`) is in its final sorted position. The output of the program will be:

The fifth element is 5

Note that the other elements in the vector are not sorted, but only the fifth element is guaranteed to be in its final position.