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 the following representations of the linked list.
Java
// Linked List Classclass LinkedList{ // Head of list Node head; // 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.
Add a node at the front: (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 a node at the front.
Java
/* 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 a constant amount of work.
Add a node after a given node: (5 steps process)
We are given a pointer to a node, and the new node is inserted after the given node.
Java
/* 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 a 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 the end and then change the next to last node to a new node.
Following are the 6 steps to add node at the end.
Java
/* 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 the 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 the tail of the linked list/
Following is a complete program that uses all of the above methods to create a linked list.
Java
// A complete working Java program to// demonstrate all insertion methods// on linked listclass LinkedList{ // head of list Node head; // 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 code 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( "Created Linked list is: "); llist.printList(); }}// This code is contributed by Rajat Mishra |
Output:
Created Linked list is: 1 7 8 6 4
Time complexity: O(N) where N is size of given linked list
Auxiliary space: O(1) constant space is being used
Please refer complete article on Linked List | Set 2 (Inserting a node) for more details!
