difference between O(n) and O(3n) is only about micro-optimizations. Meaning you cannot have any general rule about that. That's precisely why, when talking about time complexity, you're always reducing O(3n) to O(n).

For example, doing only 1 pass, you'll generally need extra condition checks. That takes time... => That slowes down the O(n) implemetation... => That's microoptimization matter... => That's pointless to make general comparisons between O(n) and O(3n) because behaviors will depend on compiler, implementation, interpreter if any, ..., ..., ..., ...

Big O notation is cool, but n is still way better than 3n.

This solution seems to be missing an empty string check, thus potential undefined behaviour: https://en.cppreference.com/w/cpp/string/basic_string/pop_back

Ah. You're right. I feel blind.

Creating the list is still O(N)

Thank you for the tip!

Since c++11 it's allowed to use simpler initialisation lists for (amongst others) maps, so instead of std::make_pair('A', 'T') you may use just {'A', 'T'}

nice I do not know why i did not think of that .... will get better over time i guess