So next in the algorithm section we are going to write a linked list using python. If you don’t know what is linked list you can read it here.

# Algorithms: Coding a linked list in python

**A linked list** is a linear collection of data elements, called nodes, each pointing to the next node by means of a pointer. It is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of data and a reference (in other words, a *link*) to the next node in the sequence.

The above is a representation of linked list.

For linked list we need a class node which represent each data elementĀ in linked list. The class will be like below

class node: def __init__(self, data=None, next_node=None): self.data = data self.next_node = next_node

Here the class contains data element, next_node in the __init__ function. You can also write a getter setter function to make it accessible by calling functions.

Next we need a class for linked list which will hold the head of the linked list. Look at the class below.

class linked_list: def __init__(self, head = None): self.head = head

We will now write functions to insert, delete, search and get size of the linked list.

### Insertion

def insert_at_start(self, data): tmpnode = node(data) tmpnode.next_node = self.head self.head = tmpnode def insert_at_end(self, data): tmpnode = node(data) current = self.head while current.next_node: current = current.next_node current.next_node = tmpnode

The first function is used to insert at the start and second function for inserting at the end.

#### How does these work.

Insert at Start: Here to initialize a node with data element. Then assign the next_node as head of the linked list and head as the newly created linked list. Thus resulting in insertion at the start.

Insert at End: Here we initialize a node, traverse to the end of the linked list and assign current.next_node to the newly created one.

### Search

def search(self, data): current = self.head found = False while current and found is False: if current.data == data: found = True else: current = current.next_node if current is None: current= {} current.data = "Not found" return current

Here we traverse the list and search for the data present on any node. Else returning Not found in current.data.

### Delete:

def delete(self, data): current = self.head if current.data == data: self.head = current.next_node while current.next_node: if current.next_node.data == data: current.next_node = current.next_node.next_node else: current = current.next_node

I am leaving this one and other functions for you to understand. Below is the whole code. Have a look try to understand and comment if you don’t understand.

class node: def __init__(self, data=None, next_node=None): self.data = data self.next_node = next_node class linked_list: def __init__(self, head = None): self.head = head def insert_at_start(self, data): tmpnode = node(data) tmpnode.next_node = self.head self.head = tmpnode def insert_at_end(self, data): tmpnode = node(data) current = self.head while current.next_node: current = current.next_node current.next_node = tmpnode def search(self, data): current = self.head found = False while current and found is False: if current.data == data: found = True else: current = current.next_node if current is None: print "Data not in list" return current def size(self): current = self.head count = 0 while current: count += 1 current = current.next_node return count def traversal(self): current = self.head while current.next_node: print current.data current = current.next_node print current.data def delete(self, data): current = self.head if current.data == data: self.head = current.next_node while current.next_node: if current.next_node.data == data: current.next_node = current.next_node.next_node else: current = current.next_node a = linked_list() a.insert_at_start(1) a.insert_at_start(2) a.insert_at_start(3) a.insert_at_start(4) a.insert_at_end(5) a.traversal() # 4 # 3 # 2 # 1 # 5 print a.search(1).data # 1 a.delete(1) print 'deleted 1' a.traversal() # 4 # 3 # 2 # 5 a.delete(5) print 'deleted 5' a.traversal() # 4 # 3 # 2 print a.size() # 3

Try running the code and modify it to find out mistake and comment if you find one. I know there is one and I was too lazy to fix that š

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