Create PartitionList.py

Author: careermonk

src/chapter03linkedlists/PartitionList.py

@@ -0,0 +1,50 @@
+# Copyright (c) Dec 22, 2014 CareerMonk Publications and others.
+# E-Mail           		: [email protected] 
+# Creation Date    		: 2014-01-10 06:15:46 
+# Last modification		: 2008-10-31 
+#               by		: Narasimha Karumanchi 
+# Book Title			: Data Structures And Algorithms Made In Java
+# Warranty         		: This software is provided "as is" without any 
+# 				   warranty; without even the implied warranty of 
+# 				    merchantability or fitness for a particular purpose. 
+
+# Node of a Singly Linked List
+class Node:
+    def __init__(self, data):
+        self.data = data
+        self.next = None
+
+# class for defining a linked list   
+class PartitionList(object):
+    # initializing a list
+    def __init__(self):
+        self.length = 0
+        self.head = None
+    def partition(self, head, X):
+        # lesser and greater are the two pointers used to create two list
+        # lesser_head and greater_head are used to save the heads of the two lists.
+        # All of these are initialized with the dummy nodes created.
+        lesser = lesser_head = Node(0)
+        greater = greater_head = Node(0)
+
+        while head:
+            # If the original list node is lesser than the given X, assign it to the lesser list.
+            if head.data < X:
+                lesser.next = head
+                lesser = lesser.next
+            else:
+                # If the original list node is greater or equal to the given X, assign it to the greater list.
+                greater.next = head
+                greater = greater.next
+
+            # move ahead in the original list
+            head = head.next
+
+        # Last node of "greater" list would also be ending node of the reformed list
+        greater.next = None
+
+        # Once all the nodes are correctly assigned to the two lists,
+        # combine them to form a single list which would be returned.
+        lesser.next = greater_head.next
+
+        return lesser_head.next