cf1019 B. The hat（交互题，二分）

见这篇题解

https://blog.csdn.net/weixin_43960414/article/details/104169784

int n;

int ask(int i) {
    ll a1, a2;
    cout << "? " << i << endl; cin >> a1;
    cout << "? " << i+n/2 << endl; cin >> a2;
    return a1 - a2;
}

signed main() {
    cin >> n;
    int ans0 = ask(1);
    if(ans0 == 0) return cout << "! 1" << endl, 0;
    if(ans0 & 1) return cout << "! -1" << endl, 0;
    int l = 1, r = n/2;
    while(l < r) {
        int mid = l + r >> 1, ans1 = ask(mid);
        if(ans1 == 0) return cout << "! " << mid << endl, 0;
        if(ans1 & 1) return cout << "! -1" << endl, 0;
        if(ans1 * ans0 < 0) r = mid; else l = mid + 1;
    }
    cout << "! " << l << endl;
}