Python Program to Perform Singly Linked List Operations

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This is a Python program to illustrate the operations of a singly linked list.

Problem Description

The program creates a singly linked list and presents the user with a menu to perform various operations on the list.

Problem Solution

1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variable head.
3. The variable head points to the first element in the singly linked list.
4. Define methods get_node, get_prev_node, insert_after, insert_before, insert_at_beg, insert_at_end, remove and display.
5. get_node takes an index as argument and traverses the list from the first node that many times to return the node at that index.
6. get_prev_node takes a reference node as argument and returns the previous node. It returns None when the reference node is the first node.
7. The methods insert_after and insert_before insert a node after or before some reference node in the list.
8. The methods insert_at_beg and insert_at_end insert a node at the first or last position of the list.
9. The method remove takes a node as argument and removes it from the list.
10. The method display traverses the list from the first node and prints the data of each node.
11. Create an instance of LinkedList and present the user with a menu to perform operations on the list.

Program/Source Code

Here is the source code of a Python program to create a singly linked list and illustrate its operations. The program output is shown below.

class Node:
    def __init__(self, data):
       self.data = data
       self.next = None
 
 
class LinkedList:
    def __init__(self):
        self.head = None
 
    def get_node(self, index):
        current = self.head
        for i in range(index):
            if current is None:
                return None
            current = current.next
        return current
 
    def get_prev_node(self, ref_node):
        current = self.head
        while (current and current.next != ref_node):
            current = current.next
        return current
 
    def insert_after(self, ref_node, new_node):
        new_node.next = ref_node.next
        ref_node.next = new_node
 
    def insert_before(self, ref_node, new_node):
        prev_node = self.get_prev_node(ref_node)
        self.insert_after(prev_node, new_node)
 
    def insert_at_beg(self, new_node):
        if self.head is None:
            self.head = new_node
        else:
            new_node.next = self.head
            self.head = new_node
 
    def insert_at_end(self, new_node):
        if self.head is None:
            self.head = new_node
        else:
            current = self.head
            while current.next is not None:
                current = current.next
            current.next = new_node
 
    def remove(self, node):
        prev_node = self.get_prev_node(node)
        if prev_node is None:
            self.head = self.head.next
        else:
            prev_node.next = node.next
 
    def display(self):
        current = self.head
        while current:
            print(current.data, end = ' ')
            current = current.next
 
 
a_llist = LinkedList()
 
print('Menu')
print('insert <data> after <index>')
print('insert <data> before <index>')
print('insert <data> at beg')
print('insert <data> at end')
print('remove <index>') 
print('quit')
 
while True:
    print('The list: ', end = '')
    a_llist.display()
    print()
    do = input('What would you like to do? ').split()
 
    operation = do[0].strip().lower()
 
    if operation == 'insert':
        data = int(do[1])
        position = do[3].strip().lower()
        new_node = Node(data)
        suboperation = do[2].strip().lower() 
        if suboperation == 'at':
            if position == 'beg':
                a_llist.insert_at_beg(new_node)
            elif position == 'end':
                a_llist.insert_at_end(new_node)
        else:
            index = int(position)
            ref_node = a_llist.get_node(index)
            if ref_node is None:
                print('No such index.')
                continue
            if suboperation == 'after':
                a_llist.insert_after(ref_node, new_node)
            elif suboperation == 'before':
                a_llist.insert_before(ref_node, new_node)
 
    elif operation == 'remove':
        index = int(do[1])
        node = a_llist.get_node(index)
        if node is None:
            print('No such index.')
            continue
        a_llist.remove(node)
 
    elif operation == 'quit':
        break
Program Explanation

1. An instance of LinkedList is created.
2. The user is presented with a menu to perform various operations on the list.
3. The corresponding methods are called to perform each operation.

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Runtime Test Cases
Case 1:
Menu
insert <data> after <index>
insert <data> before <index>
insert <data> at beg
insert <data> at end
remove <index>
quit
The list: 
What would you like to do? insert 7 at beg
The list: 7 
What would you like to do? insert 3 at end
The list: 7 3 
What would you like to do? insert 1 after 0
The list: 7 1 3 
What would you like to do? insert 9 before 2
The list: 7 1 9 3 
What would you like to do? remove 2
The list: 7 1 3 
What would you like to do? insert 12 at end
The list: 7 1 3 12 
What would you like to do? remove 0
The list: 1 3 12 
What would you like to do? quit
 
Case 2:
Menu
insert <data> after <index>
insert <data> before <index>
insert <data> at beg
insert <data> at end
remove <index>
quit
The list: 
What would you like to do? insert 5 after 0
No such index.
The list: 
What would you like to do? insert 3 at end
The list: 3 
What would you like to do? insert 1 after 0
The list: 3 1 
What would you like to do? insert 2 before 1
The list: 3 2 1 
What would you like to do? insert 0 at end
The list: 3 2 1 0 
What would you like to do? remove 3
The list: 3 2 1 
What would you like to do? remove 2
The list: 3 2 
What would you like to do? quit

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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