Add policy tree

Signed-off-by: bruno.tissei <bruno.tissei@arculus.de>

structure/policy_tree.cpp

+/**
+ * Policy Tree
+ *
+ * Complexity (Time):
+ *   insert        -> O(log n)
+ *   erase         -> O(log n)
+ *   find_by_order -> O(log n)
+ *   order_of_key  -> O(log n)
+ * Complexity (Space): O(n)
+ */
+
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+
+using namespace __gnu_pbds;
+
+typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> set_t;
+
+void operations() {
+  set_t S;
+
+  // Insert element in S
+  S.insert(x);
+
+  // Remove element from S
+  S.erase(x);
+
+  // Returns iterator to the k-th largest element (counting from zero)
+  int pos = *S.find_by_order(k);
+
+  // Returns the number of items strictly smaller than x
+  int ord = S.order_of_key(x)
+}