C++ String Interview Question

Question

I was recently in a C++ technical interview, where I was given a bit of simple string manipulation code, which is intended to take a string and return a string that is comprised

Answer 1

If you must pass the tests then you're going to have to write inefficient code, because you must return a copy of a string. This implies you must use dynamic allocation, possibly multiple times because of the copy.

So change the tests and change the signature.

template
Out first_last_n(const std::string::size_type& n, const std::string& s, Out r)
{
    r = copy_n(s.begin(), n, r);
    std::string::const_iterator pos(s.end());
    std::advance(pos, -n);
    return copy_n(pos, n, r);
}

Then call it like so:

std::string s("Hello world!");
char r[5];
r[4] = 0;
first_last_n(2, s, r);

This allows you to use dynamic programming, and it eliminates the need of dynamic allocation in the function.

I like my algorithms minimalistic, and I purposely eliminated the check for n being smaller or equal to the size of the string. I replace the check with a pre-condition for the function. Preconditions are faster than checks: they have zero overhead.