Python Program For Insertion Sort In A Singly Linked List

We have discussed Insertion Sort for arrays. In this article we are going to discuss Insertion Sort for linked list. 
Below is a simple insertion sort algorithm for a linked list. 

1) Create an empty sorted (or result) list.
2) Traverse the given list, do following for every node.
......a) Insert current node in sorted way in sorted or result list.
3) Change head of given linked list to head of sorted (or result) list.

The main step is (2.a) which has been covered in the post Sorted Insert for Singly Linked List Below is implementation of above algorithm:

Python

# Python implementation of above
# algorithm
# Node class
class Node:

    # Constructor to initialize the
    # node object
    def __init__(self, data):
        self.data = data
        self.next = None

# Function to sort a singly
# linked list using insertion
# sort
def insertionSort(head_ref):

    # Initialize sorted linked list
    sorted = None

    # Traverse the given linked list
    # and insert every node to sorted
    current = head_ref
    while (current != None):

        # Store next for next iteration
        next = current.next

        # Insert current in sorted
        # linked list
        sorted = sortedInsert(sorted,
                              current)
        # Update current
        current = next

    # Update head_ref to point to
    # sorted linked list
    head_ref = sorted
    return head_ref

# Function to insert a new_node in a list.
# Note that this function expects a pointer
# to head_ref as this can modify the head
# of the input linked list (similar to push())
def sortedInsert(head_ref, new_node):
    current = None

    # Special case for the head end
    if (head_ref == None or
       (head_ref).data >= new_node.data):
        new_node.next = head_ref
        head_ref = new_node

    else:

        # Locate the node before the point
        # of insertion
        current = head_ref
        while (current.next != None and
               current.next.data < new_node.data):
            current = current.next

        new_node.next = current.next
        current.next = new_node

    return head_ref

# Utility Functions
# Function to print linked list
def printList(head):
    temp = head
    while(temp != None):
        print( temp.data, end = " ")
        temp = temp.next

# A utility function to insert a node
# at the beginning of linked list
def push( head_ref, new_data):

    # Allocate node
    new_node = Node(0)

    # Put in the data
    new_node.data = new_data

    # Link the old list of the
    # new node
    new_node.next = (head_ref)

    # Move the head to point to
    # the new node
    (head_ref) = new_node
    return head_ref

# Driver code
a = None
a = push(a, 5)
a = push(a, 20)
a = push(a, 4)
a = push(a, 3)
a = push(a, 30)

print("Linked List before sorting ")
printList(a)

a = insertionSort(a)

print("Linked List after sorting ")
printList(a)
# This code is contributed by Arnab Kundu

Java

// Java implementation of the above algorithm
// Node class
class Node {
    int data;
    Node next;

    // Constructor to initialize the node object
    public Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}

public class InsertionSortLinkedList {

    // Function to sort a singly linked list using insertion
    // sort
    public static Node insertionSort(Node head)
    {

        // Initialize sorted linked list
        Node sorted = null;

        // Traverse the given linked list and insert every
        // node to sorted
        Node current = head;
        while (current != null) {

            // Store next for next iteration
            Node next = current.next;

            // Insert current in sorted linked list
            sorted = sortedInsert(sorted, current);

            // Update current
            current = next;
        }

        // Return head_ref to point to sorted linked list
        return sorted;
    }

    // Function to insert a new_node in a list.
    // Note that this function expects a pointer to head_ref
    // as this can modify the head of the input linked list
    // (similar to push())
    public static Node sortedInsert(Node head,
                                    Node new_node)
    {
        Node current = null;

        // Special case for the head end
        if (head == null || head.data >= new_node.data) {
            new_node.next = head;
            head = new_node;
        }
        else {

            // Locate the node before the point of insertion
            current = head;
            while (current.next != null
                   && current.next.data < new_node.data) {
                current = current.next;
            }

            new_node.next = current.next;
            current.next = new_node;
        }

        return head;
    }

    // Utility Functions

    // Function to print linked list
    public static void printList(Node head)
    {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
    }

    // A utility function to insert a node at the beginning
    // of linked list
    public static Node push(Node head, int new_data)
    {

        // Allocate node
        Node new_node = new Node(new_data);

        // Link the old list of the new node
        new_node.next = head;

        // Move the head to point to the new node
        head = new_node;
        return head;
    }

    // Driver code
    public static void main(String[] args)
    {
        Node a = null;
        a = push(a, 5);
        a = push(a, 20);
        a = push(a, 4);
        a = push(a, 3);
        a = push(a, 30);

        System.out.println("Linked List before sorting ");
        printList(a);

        a = insertionSort(a);

        System.out.println("\nLinked List after sorting ");
        printList(a);
    }
}

Output:

Linked List before sorting
30  3  4  20  5
Linked List after sorting
3  4  5  20  30

Time Complexity: O(n²), in the worst case, we might have to traverse all nodes of the sorted list for inserting a node, and there are “n” such nodes.

Auxiliary Space : O(1), no extra space is required depending on the size of the input, thus it is constant.

Please refer complete article on Insertion Sort for Singly Linked List for more details!