We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.
All programs discussed in this post consider following representations of linked list
Java Programming:
// Linked List Class
class LinkedList
{
Node head; // head of list
/* Node Class */
class Node
{
int data;
Node next;
// Constructor to create a new node
Node(int d) {data = d; next = null; }
}
}
In this post, methods to insert a new node in linked list are discussed. A node can be added in three ways
1) At the front of the linked list
2) After a given node.
3) At the end of the linked list.
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Add a node at the front: (A 4 steps process)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)
Following are the 4 steps to add node at the front.
Java Programming:
/* This function is in LinkedList class. Inserts a
new Node at front of the list. This method is
defined inside LinkedList class shown above */
public void push(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/
Node new_node = new Node(new_data);
/* 3. Make next of new Node as head */
new_node.next = head;
/* 4. Move the head to point to new Node */
head = new_node;
}
Time complexity of push() is O(1) as it does constant amount of work.
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Add a node after a given node: (5 steps process)
We are given pointer to a node, and the new node is inserted after the given node.
Java Programming:
/* This function is in LinkedList class.
Inserts a new node after the given prev_node. This method is
defined inside LinkedList class shown above */
public void insertAfter(Node prev_node, int new_data)
{
/* 1. Check if the given Node is null */
if (prev_node == null)
{
System.out.println("The given previous node cannot be null");
return;
}
/* 2. Allocate the Node &
3. Put in the data*/
Node new_node = new Node(new_data);
/* 4. Make next of new Node as next of prev_node */
new_node.next = prev_node.next;
/* 5. make next of prev_node as new_node */
prev_node.next = new_node;
}
Time complexity of insertAfter() is O(1) as it does constant amount of work.
Add a node at the end: (6 steps process)
The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30.
Since a Linked List is typically represented by the head of it, we have to traverse the list till end and then change the next of last node to new node.
Following are the 6 steps to add node at the end
Java Programming:
/* Appends a new node at the end. This method is
defined inside LinkedList class shown above */
public void append(int new_data)
{
/* 1. Allocate the Node &
2. Put in the data
3. Set next as null */
Node new_node = new Node(new_data);
/* 4. If the Linked List is empty, then make the
new node as head */
if (head == null)
{
head = new Node(new_data);
return;
}
/* 4. This new node is going to be the last node, so
make next of it as null */
new_node.next = null;
/* 5. Else traverse till the last node */
Node last = head;
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
return;
}
Time complexity of append is O(n) where n is the number of nodes in linked list. Since there is a loop from head to end, the function does O(n) work.
This method can also be optimized to work in O(1) by keeping an extra pointer to tail of linked list
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Following is a complete program that uses all of the above methods to create a linked list.
Java Programming:
// A complete working Java program to demonstrate all insertion methods
// on linked list
class LinkedList
{
Node head; // head of list
/* Linked list Node*/
class Node
{
int data;
Node next;
Node(int d) {data = d; next = null; }
}
/* Inserts a new Node at front of the list. */
public void push(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/
Node new_node = new Node(new_data);
/* 3. Make next of new Node as head */
new_node.next = head;
/* 4. Move the head to point to new Node */
head = new_node;
}
/* Inserts a new node after the given prev_node. */
public void insertAfter(Node prev_node, int new_data)
{
/* 1. Check if the given Node is null */
if (prev_node == null)
{
System.out.println("The given previous node cannot be null");
return;
}
/* 2 & 3: Allocate the Node &
Put in the data*/
Node new_node = new Node(new_data);
/* 4. Make next of new Node as next of prev_node */
new_node.next = prev_node.next;
/* 5. make next of prev_node as new_node */
prev_node.next = new_node;
}
/* Appends a new node at the end. This method is
defined inside LinkedList class shown above */
public void append(int new_data)
{
/* 1. Allocate the Node &
2. Put in the data
3. Set next as null */
Node new_node = new Node(new_data);
/* 4. If the Linked List is empty, then make the
new node as head */
if (head == null)
{
head = new Node(new_data);
return;
}
/* 4. This new node is going to be the last node, so
make next of it as null */
new_node.next = null;
/* 5. Else traverse till the last node */
Node last = head;
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
return;
}
/* This function prints contents of linked list starting from
the given node */
public void printList()
{
Node tnode = head;
while (tnode != null)
{
System.out.print(tnode.data+" ");
tnode = tnode.next;
}
}
/* Driver program to test above functions. Ideally this function
should be in a separate user class. It is kept here to keep
code compact */
public static void main(String[] args)
{
/* Start with the empty list */
LinkedList llist = new LinkedList();
// Insert 6. So linked list becomes 6->NUllist
llist.append(6);
// Insert 7 at the beginning. So linked list becomes
// 7->6->NUllist
llist.push(7);
// Insert 1 at the beginning. So linked list becomes
// 1->7->6->NUllist
llist.push(1);
// Insert 4 at the end. So linked list becomes
// 1->7->6->4->NUllist
llist.append(4);
// Insert 8, after 7. So linked list becomes
// 1->7->8->6->4->NUllist
llist.insertAfter(llist.head.next, 8);
System.out.println("\nCreated Linked list is: ");
llist.printList();
}
}
// This code is contributed by Rajat Mishra
Output:
Created Linked list is: 1 7 8 6 4