for (char c : s) {if (c == 'a') {cnt++;} else {cnt = max(cnt - 1, 0ll);}
}

23ccpc桂林GProblem - G - Codeforces

void solve() {std::string s;std::cin >> s;int cnt = 0;for (char c : s) {if (c == '(') {cnt++;}else {cnt = std::max(cnt - 1, 0);}}std::cout << (cnt ? "impossible" : s) << '\n';
}signed main() {std::ios::sync_with_stdio(false);std::cin.tie(0), std::cout.tie(0);int t; std::cin >> t; while (t--) {solve();}return 0;
}

好串登录—专业IT笔试面试备考平台_牛客网

using ll=long long;
void solve() {std::string s;std::cin >> s;ll cnt = 0,x=0,y=0;for (char c : s) {if (c == 'a') {cnt++;x++;}else {cnt = std::max(cnt - 1LL, 0LL);y++;}}if(s[0]=='b'||x!=y)cnt=1LL;std::cout << (cnt ? "Bad" : "Good") << '\n';
}signed main() {std::ios::sync_with_stdio(false);std::cin.tie(0), std::cout.tie(0);int t=1; while (t--) {solve();}return 0;
}