Adiciona k-elements

fontes/k-elements.h

+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+using namespace __gnu_pbds;
+template <class T, class U = less<T>>
+using ord_set = tree<T, null_type, U, rb_tree_tag,
+      tree_order_statistics_node_update>;
+template <class C = less<>>
+struct k_elements {
+  int k; ord_set<pair<ll, int>, C> s; ll sum;
+  k_elements(int k) : k(k), sum(0) {};
+  void insert(int key, ll value) {
+    int l = s.order_of_key(make_pair(value, key));
+    s.insert(make_pair(value, key));
+    if (l >= k) { return; }
+    sum += value;
+    if (s.size() > k)
+      sum -= s.find_by_order(k)->first;
+  }
+  void remove(int key, ll value) {
+    if (s.order_of_key(make_pair(value, key)) < k) {
+      sum -= value;
+      if (s.size() > k)
+        sum += s.find_by_order(k)->first;
+    }
+    s.erase(s.find(make_pair(value, key)));
+  }
+};