Decrease on number from string

void minusone (){

    for (ll i = a.size() - 1; i >= 0; i--){
        if (a[i] == '0') continue;
        a[i]--;
        for (ll j = i + 1; j < a.size(); j++){
            a[j] = '9';
        }

        return;
    }
}