Add Multiset data structure (#538)

pydatastructs/miscellaneous_data_structures/__init__.py

@@ -45,3 +45,8 @@
     RangeQueryDynamic
 )
 __all__.extend(algorithms.__all__)
+
+from .multiset import (
+    Multiset
+)
+__all__.extend(multiset.__all__)

pydatastructs/miscellaneous_data_structures/multiset.py

@@ -0,0 +1,42 @@
+__all__ = [
+    'Multiset'
+]
+
+
+class Multiset:
+    def __init__(self, *args):
+        # TODO: Implement dict in pydatastructs
+        self.counter = dict()
+        from pydatastructs.trees import RedBlackTree
+        self.tree = RedBlackTree()
+        self._n = 0
+        for arg in args:
+            self.add(arg)
+
+    def add(self, element):
+        self.counter[element]  = self.counter.get(element, 0) + 1
+        self._n += 1
+        if self.counter[element] == 1:
+            self.tree.insert(element)
+
+    def remove(self, element):
+        if self.counter[element] == 1:
+            self.tree.delete(element)
+        if self.counter.get(element, 0) > 0:
+            self._n -= 1
+            self.counter[element] -= 1
+
+    def lower_bound(self, element):
+        return self.tree.lower_bound(element)
+
+    def upper_bound(self, element):
+        return self.tree.upper_bound(element)
+
+    def __contains__(self, element):
+        return self.counter.get(element, 0) > 0
+
+    def __len__(self):
+        return self._n
+
+    def count(self, element):
+        return self.counter.get(element, 0)

pydatastructs/miscellaneous_data_structures/tests/test_multiset.py

@@ -0,0 +1,39 @@
+from pydatastructs.miscellaneous_data_structures import Multiset
+
+def test_Multiset():
+
+    ms = Multiset()
+    ms.add(5)
+    ms.add(5)
+    ms.add(3)
+    ms.add(7)
+    assert len(ms) == 4
+    assert 5 in ms
+    assert ms.count(5) == 2
+    assert ms.count(3) == 1
+    assert ms.count(-3) == 0
+    assert not 4 in ms
+    ms.remove(5)
+    assert 5 in ms
+    assert ms.lower_bound(5) == 5
+    assert ms.upper_bound(5) == 7
+
+    ms = Multiset(5, 3, 7, 2)
+
+    assert len(ms) == 4
+    assert 5 in ms
+    assert ms.count(7) == 1
+    assert not 4 in ms
+    assert ms.lower_bound(3) == 3
+    assert ms.upper_bound(3) == 5
+    assert ms.upper_bound(7) is None
+
+    ms.remove(5)
+
+    assert len(ms) == 3
+    assert not 5 in ms
+
+    ms.add(4)
+
+    assert 4 in ms
+    assert len(ms) == 4