出题人的警示后人 & 解释 UKE 的原因

      for (int i = 1; i <= n - 1; i++){
            int a = ouf.readInt(), b = ouf.readInt();
            if (a == -1) quitf(_wa, "There is a solution but participant did not found in testcase");
            if (st.find(a) == st.end()) quitf(_wa, "In testcase %d: Element %d not found", tc, a);
            else if (st.find(b) == st.end()) quitf(_wa, "In testcase %d: Element %d not found.", tc, b);
            else{
                int cur = mex(a, b);
                st.erase(st.find(a));
                st.erase(st.find(b));
                st.insert(cur);
            }
        }