linked_list.py: Add Linked List Data Structure

This commit adds the code for Linked List Data Structure in Python
which has various common methods for inserting and deleting elements

Closes #25

README.md

@@ -30,7 +30,7 @@ This repository contains examples of various algorithms written on different pro
 |:----------------------------------------------------------------------------------------------- |:-------------------------------------:|:-------------------------------------:|:-------------------------------------:|:-------------------------------------:|
 | [Queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type))                               |                                       | [:octocat:](queue/Cpp)                |                                       |                                       |
 | [Stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type))                               |                                       |                                       | [:octocat:](stack/Java)               | [:octocat:](stack/Python)             |
-| [Linear Linked List](https://en.wikipedia.org/wiki/Linked_list)                                 | [:octocat:](linked_list/C)            |                                       |                                       |                                       |
+| [Linear Linked List](https://en.wikipedia.org/wiki/Linked_list)                                 | [:octocat:](linked_list/C)            |                                       |                                       | [:octocat:](linked_list/Python)       |
 | [AVL Tree](https://en.wikipedia.org/wiki/AVL_tree)                                              | [:octocat:](avl_tree/C)               |                                       | [:octocat:](avl_tree/Java)            | [:octocat:](avl_tree/Python)          |
 
 

linked_list/Python/linked_list.py

@@ -0,0 +1,176 @@
+#!/usr/bin/env python3
+
+
+class NoSuchNodeException(Exception):
+    """Exception class"""
+    pass
+
+
+class Node:
+    """
+    Class for the structure of node in Linked List
+
+    :param data:    Data value for node
+    :param next_node:   Next node reference
+    """
+
+    def __init__(self, data=None, next_node=None):
+        self.data = data
+        self.next_node = next_node
+
+    def compare_to(self, node):
+        """
+        Method to compare node data
+
+        :return:    Returns True if node data matches
+        """
+        return node.data == self.data
+
+
+class LinkedList:
+    """
+    Class for the Linked List
+    :param head:    Reference of the head.
+    :param list_size:   List size.
+    """
+
+    def __init__(self, head=None, list_size=0):
+        self.head = head
+        self.list_size = list_size
+
+    def size(self):
+        """
+        This methods returns the size of the linked list.
+
+        :return:    The size of the linked list.
+        """
+        return self.list_size
+
+    def is_empty(self):
+        """
+        This methods determines whether the list is empty.
+        :return:    True if the list is empty, otherwise False.
+        """
+        return self.list_size == 0
+
+    def get_first(self):
+        """
+        This method returns the first element of the list.
+
+        :return:    The first element of the list.
+        """
+        if self.is_empty():
+            raise NoSuchNodeException()
+
+        return self.head.data
+
+    def getNode(self, i):
+        """
+        This method takes an index and finds the node residing.
+
+        :raises:    NoSuchNodeException if no such element.
+        :param i:   The index of the node.
+        :return:    The ith node of the list.
+        """
+        if i < 0 or i > self.list_size - 1:
+            raise NoSuchNodeException()
+
+        current = 0
+        p = self.head
+
+        while current < i:
+            p = p.next_node
+            current += 1
+
+        return p
+
+    def add_first(self, data):
+        """
+        This method adds a node to the front of the list.
+
+        :param data:    The data of the node to add.
+        """
+        node = Node(data, self.head)
+        self.head = node
+
+        self.list_size += 1
+
+    def add_last(self, data):
+        """
+        This method adds a node to the back of the list.
+
+        :param data:    The data of the node to add.
+        """
+        node = Node(data, None)
+
+        if self.is_empty():
+            self.head = node
+        else:
+            tail = self.getNode(self.list_size - 1)
+            tail.next_node = node
+
+        self.list_size += 1
+
+    def remove_first(self):
+        """
+        This method removes the first element of the list.
+
+        :return:    The value of the first element.
+        """
+        if self.is_empty():
+            raise NoSuchNodeException()
+
+        tmp_val = self.head.data
+        self.head = self.head.next_node
+        self.list_size -= 1
+        return tmp_val
+
+    def remove_last(self):
+        """
+        This method removes the last element of the list.
+
+        :return:    The value of the removed element.
+        """
+        if self.is_empty():
+            raise NoSuchNodeException()
+
+        tail = self.getNode(self.list_size - 1)
+        tail_data = tail.data
+
+        if self.list_size == 1:
+            self.head = None
+        else:
+            before_tail = self.getNode(self.list_size - 2)
+            before_tail.next_node = None
+
+        self.list_size -= 1
+        return tail_data
+
+    def print_list(self):
+        """
+        Method to print the list in a newline after each element.
+        """
+        p = self.head
+        i = 0
+
+        while i < self.size():
+            print(p.data)
+            i += 1
+            p = p.next_node
+
+
+def main():
+    myList = LinkedList()
+    myList.add_first(10)
+    myList.add_first(20)
+    myList.add_first(30)
+    print("Current List after adding 3 elements: ")
+    myList.print_list()
+
+    myList.remove_last()
+    print("Current List after removing last element: ")
+    myList.print_list()
+
+
+if __name__ == "__main__":
+    main()