Insertion in Linked List Data Structure | DSA Tutorials

Insertion in Linked List – In a linked list, to insert a node at any position, like at the end, first, or in between the nodes, we need a reference to the node of the list. So first we need to find the reference to the node of the list. And if you don’t know how to find the reference to the node of the list, then first read this post – Operations on the Linked list

Insertion in Linked List Data Structure

After completing this tutorial, you can 

1. Insert the node at the beginning of the linked list
2. Insert the node in an empty linked list
3. Insert the node at the end of the linked list
4. Insert the node between the nodes of the linked list
5. Insert the node after a node of the linked list
6. Insert the node before a node of the linked list
7. Insert the node at a given position in the linked list

Insert the node at the beginning of the Linked List

To insert a node at the beginning of the list first, we need to allocate a new node called temp, as you see in Figure 1.

After that, we point the linked part of the new node to the first node of the list. And then we point the start variable to the temp node. So it will become the first node of the list.

Note: the process and steps we follow are very important because if we reverse the order of steps to insert the node, then we are stuck in an infinite loop of the list.

def insert_in_beginning(self, data):
    temp = Node(data)
    temp.link = self.start
    self.start = temp

def insert_in_beginning(self, data):
    temp = Node(data)
    temp.link = self.start
    self.start = temp

Insert the node in an empty Linked List

In the empty linked list, we don’t have any nodes. So till now, our list is empty, the reference to the first node is null or None.

First, we allocate a new node called temp, and then we point the reference of the node to the start variable.

def insert_at_end(self, data):
    temp = Node(data)
    if self.start is None:
        self.start = temp

def insert_at_end(self, data):
    temp = Node(data)
    if self.start is None:
        self.start = temp

Insert the node at the end of the Linked List

To insert a node at the end of the list, first we need a reference to the last node of the list. after finding the reference to the last node of the list. First, we allocate a new node called temp.

And then we point the last node of the list to the newly allocated node.

def insert_at_end(self, data):
    temp = Node(data)
    if self.start is None:
        self.start = temp
        return
    p = self.start
    while p.link is not None:
        p = p.link
    p.link = temp

def insert_at_end(self, data):
    temp = Node(data)
    if self.start is None:
        self.start = temp
        return
    p = self.start
    while p.link is not None:
        p = p.link
    p.link = temp

Insert the node between the nodes of the Linked List

To insert a node between the nodes, we need a reference to the node after we are going to insert the node, as you see in Figure 9. And we allocate a new node called temp.

And then we store the reference of the next node of p into the temp node, as you see in Figure 10.

And then we store the reference of the temp node in the p node.

Note: the order of the steps to insert the new node between the nodes of the list is very important. If we reverse our steps, then we are stuck in an infinite loop.

Insert a node after a node of the linked list

As if we want to insert the node after a node that has value x, as you see in Figure 13.

For example, we need to insert the node after the node that has the value x=56, then first we need to find the reference to the node that has the particular value.

And then we allocate a new node called temp.

And then we store the reference of the next node to the new node, as you see in Figure 16.

Then we store the reference of the newly allocated node in the p node.

def insert_after(self, data, x):
    p = self.start
    while p is not None:
        if p.info == x:
            break        p = p.link

    if p is None:
        print(x, "not present in the list")
    else:
        temp = Node(data)
        temp.link = p.link
        p.link = temp

def insert_after(self, data, x):
    p = self.start
    while p is not None:
        if p.info == x:
            break        p = p.link

    if p is None:
        print(x, "not present in the list")
    else:
        temp = Node(data)
        temp.link = p.link
        p.link = temp

Insert a node before a node of the linked list

As if we want to insert the node before a node that has the value x. For example, we need to insert the node before the node that has the value x=45. First, we need to find the reference to the predecessor of the node that has value x.

And then we allocate a new node called temp.

And then we store the reference of the node in the new node.

And then we store the reference of the new node in the p node.

def insert_before(self, data, x):
    # If list is empty    if self.start is None:
        print("List is empty")
        return    # x is in first node , ew node is to be inserted before first node    if x == self.start.info:
        temp = Node(data)
        temp.link = self.start
        self.start = temp
        return
    # Find reference to predecessor of node contanining x    p = self.start
    while p.link is not None:
        if p.link.info == x:
            break        p = p.link

    if p.link is None:
        print(x, " not present in the list")
    else:
        temp = Node(data)
        temp.link = p.link
        p.link = temp

def insert_before(self, data, x):
    # If list is empty    if self.start is None:
        print("List is empty")
        return    # x is in first node , ew node is to be inserted before first node    if x == self.start.info:
        temp = Node(data)
        temp.link = self.start
        self.start = temp
        return
    # Find reference to predecessor of node contanining x    p = self.start
    while p.link is not None:
        if p.link.info == x:
            break        p = p.link

    if p.link is None:
        print(x, " not present in the list")
    else:
        temp = Node(data)
        temp.link = p.link
        p.link = temp

Insert a node at a given position of the linked list

If we want to insert a node at the kth position in the list. Then, first, we need to find the reference to the node at the k-1 position. For example, if we want to insert the node at the position k=4, then we find the reference to the node at the 3rd position. As you see in Figure 22.

After that, we allocate a new node called temp.

Then we store the reference of the node at the kth position, as you see in Figure 24.

Then we store the reference of the newly allocated node to the k-1 node, as you see in Figure 25.

So the new node becomes the 4th node of the list.