D - Fixed Point Guessing

https://codeforces.com/contest/1698/submission/162148239

Codeforces/1698/D/D.cpp

@@ -0,0 +1,44 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+int t, n, ans;
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    cin >> t;
+
+    while (t--) {
+        ans = 0;
+
+        cin >> n;
+
+        int l = 1, r = n;
+
+        while (l <= r) {
+            int mid = l + r >> 1;
+            cout << "? " << 1 << ' ' << mid << endl;
+
+            int cnt = 0;
+            for (int i = 1, x; i <= mid; i++) {
+                cin >> x;
+
+                if (x <= mid) cnt++;
+            }
+
+            if (cnt & 1) {
+                r = mid - 1;
+                ans = mid;
+            } else {
+                l = mid + 1;
+            }
+        }
+
+        cout << "! " << ans << endl;
+    }
+
+    return 0;
+}