make-the-string-great.cpp

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    string makeGood(string s) {
        string result;
        for (const auto& ch : s) {
            char counter_ch = islower(ch) ? toupper(ch) : tolower(ch);
            if (!result.empty() && result.back() == counter_ch) {
                result.pop_back();
            } else {
                result.push_back(ch);
            }
        }
        return result;
    }
};