I NEED TO TRY MORE

#include<bits/stdc++.h>

using namespace std;

int main() {
    string u;
    cin >> u;
    string resp = "";

    deque<char> s;

    for(int i = 0; i < u.size(); i++) {
        if(u.at(i) == 'a') {
            resp += u.at(i);
        }
        else {
            if(s.empty()) s.push_back(u.at(i));
            else if(u.at(i) < s.back()) resp += u.at(i);
            else s.push_back(u.at(i));
        }
    }
    while(!s.empty()) {
        resp += s.front();
        s.pop_back();
    }

    cout << resp << endl;
    return 0;
}