ROT13

• Time : $O(|s|)$
• Space : $O(|s|)$

#include <string>
using namespace std;
std::string rot13(const std::string& str) {
    string res = "";
    auto work = [](int x) {
        return x < 13 ? x + 13 : x - 13;
    };
    for(auto ch : str) {
        if(islower(ch)) res.push_back(work(ch-'a') + 'a');
        else if(isalpha(ch)) res.push_back(work(ch-'A') + 'A');
        else res.push_back(ch);
    }
    return res;
}