--- /dev/null
+// -*- C++ -*-
+
+// Copyright (C) 2005, 2006, 2009 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the terms
+// of the GNU General Public License as published by the Free Software
+// Foundation; either version 3, or (at your option) any later
+// version.
+
+// This library is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+// General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+
+// Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
+
+// Permission to use, copy, modify, sell, and distribute this software
+// is hereby granted without fee, provided that the above copyright
+// notice appears in all copies, and that both that copyright notice
+// and this permission notice appear in supporting documentation. None
+// of the above authors, nor IBM Haifa Research Laboratories, make any
+// representation about the suitability of this software for any
+// purpose. It is provided "as is" without express or implied
+// warranty.
+
+/**
+ * @file basic_multiset_example.cpp
+ * A basic example showing how to use multisets.
+ */
+
+
+// This example shows how to use "multisets".
+
+// In this example we build a very simple priority queue that also can
+// be queried if an entry contains (i.e., it is slightly similar to an
+// associative container as well as a priority queue). The priority
+// queue adapts a "multiset".
+
+// (Note that there are more efficient ways for implementing this than
+// by adapting an associative container. This is just an example for
+// "multisets".)
+
+#include <iostream>
+#include <cassert>
+#include <ext/pb_ds/assoc_container.hpp>
+
+using namespace std;
+using namespace __gnu_pbds;
+
+// A simple priority queue that also supports an "contains" query.
+class contains_pq
+{
+public:
+ // Pushes an integer.
+ void
+ push(int i);
+
+ // Pops the largest integer and returns it.
+ int
+ pop();
+
+ // Returns true iff i is contained in the container.
+ bool
+ contains(int i) const
+ { return m_tree.find(i) != m_tree.end(); }
+
+ // Returns true iff empty.
+ bool
+ empty() const
+ { return m_tree.empty(); }
+
+private:
+ // This is the container type we adapt - a "multiset".
+ // It maps each integer to the number of times it logically appears.
+ typedef
+ tree<
+ int,
+ size_t,
+ greater<
+ int> >
+ tree_t;
+
+private:
+ tree_t m_tree;
+};
+
+void
+contains_pq::
+push(int i)
+{
+ // To push i, we insert to the "multiset" that i appears 0 times
+ // (which is a no-op if i already is contained), then increment the
+ // number of times i is contained by 1.
+ ++m_tree.insert(make_pair(i, 0)).first->second;
+}
+
+int
+contains_pq::
+pop()
+{
+ assert(!empty());
+
+ // The element we need to pop must be the first one, since tree_t is
+ // an ordered container.
+ tree_t::iterator it = m_tree.begin();
+
+ const int i = it->first;
+
+ // Decrease the number of times the popped element appears in the
+ // container object. If it is 0 - we erase it.
+ if (--it->second == 0)
+ m_tree.erase(it);
+
+ return i;
+}
+
+int main()
+{
+ contains_pq cpq;
+
+ // First we push some elements.
+ cpq.push(4);
+ cpq.push(3);
+ cpq.push(2);
+ cpq.push(1);
+ cpq.push(4);
+
+ // Note that logically, 4 appears 2 times, and each of 1, 2, and 3
+ // appear once.
+ assert(cpq.contains(4));
+ assert(cpq.contains(3));
+ assert(cpq.contains(2));
+ assert(cpq.contains(1));
+
+ // Now pop the topmost element - it should be 4.
+ assert(cpq.pop() == 4);
+
+ // Now logically, each of 1, 2, 3, and 4 appear once.
+ assert(cpq.contains(4));
+
+ // We pop the topmost element - it should be 4.
+ assert(cpq.pop() == 4);
+
+ // 4 should not be contained any more.
+ assert(!cpq.contains(4));
+
+ assert(cpq.contains(3));
+ assert(cpq.contains(2));
+ assert(cpq.contains(1));
+
+ assert(cpq.pop() == 3);
+ assert(cpq.pop() == 2);
+ assert(cpq.pop() == 1);
+
+ assert(cpq.empty());
+
+ return 0;
+}
+
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