Single Linked List

Single Linked List

Generally Linked list means a singly linked list. This list consists of a number of nodes in which each node has a next pointer to the following element. The link of the last node in the list is NULL, which indicates the end of the list.

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In any single linked list, the individual element is called as “Node”. Every “Node” contains two fields, data and next (link) . The data field is used to store actual value of that node and next field is used to store the address of the next node in the sequence. The graphical representation of a node in a single linked list is as follows

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1. In a single linked list, the address of the first node is always stored in a reference node known as “front” (Sometimes it is also known as “head”).
2. Always next part (reference part) of the last node must be NULL.

Basic Operations on a List
  1. Traversing the list
  2. Inserting an item in the list
  3. Deleting an item from the list
1. Traversing the Linked List

Let us assume that the head points to the first node of the list. To traverse the list, we do the following
1. Follow the pointers.
2. Display the contents of the nodes (or count) as they are traversed.
3. Stop when the next pointer points to NULL.

The ListLength() function takes a linked list as input and counts the number of nodes in the list. The function given below can be used for printing the list data with extra print function. 

public class ListTraverse { 
  public int ListLength(ListNode headNode) { 
    int length = 0; 
    ListNode currentNode = headNode; 
    while(currentNode != null) { 
      length ++; 
      currentNode = currentNode.getNext(); 
    } 
  return length; 
  } 
}
2.  Inserting an item in the list

In a single linked list, the insertion operation can be performed in three ways. They are as follows.

  1. Inserting at Beginning of the list
  2. Inserting at End of the list
  3. Inserting at Specific location in the list
1. Inserting at Beginning of the list

In this case, a new node is inserted before the current head node. Only one next pointer needs to be modified (new node’s next pointer) and it can be done in two steps:

  • Update the next pointer of new node, to point to the current head.
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  • Update head pointer to point to the new node.
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The following function can be used for Inserting at Beginning of the list.

public synchronized void insertAtBegin (ListNode node) { 
   node.setNext(head); 
   head = node; 
   length ++; 
}
2. Inserting a Node in Singly Linked List at the Ending

In this case, we need to modify two next pointers (last nodes next pointer and new nodes next pointer).

  • New nodes next pointer points to NULL.
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  • Last nodes next pointer points to the new node.
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3. Inserting a Node in Singly Linked List at the Middle.

Let us assume that we are given a position where we want to insert the new node. In this case also, we need to modify two next pointers.

  • If we want to add an element at position 3 then we stop at position 2. That means we traverse 2 nodes and insert the new node. For simplicity let us assume that the second node is called position node. The new node points to the next node of the position where we want to add this node.
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  • Position node’s next pointer now points to the new node.
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Time Complexity: O (n). Since, in the worst we may need to insert the node at end of the list. 

Space Complexity: O (1), for creating one temporary variable.

3. Singly Linked List Deletion

Similar to insertion, here also we have 3 cases.

  1. Deleting the first node
  2. Deleting the last node
  3. Deleting an intermediate node.
1. Deleting the First Node in Singly Linked List

First node (current head node) is removed from the list. It can be done in 2steps:

  • Create a temporary node which will point to the same node as that of head.
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  • Now, move the head nodes pointer to the next node and dispose of the temporary node.
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2. Deleting the Last Node in Singly Linked List

In this case, the last node is removed from the list. This operation is a bit trickier than removing the first node, because the algorithm should find a node, which is previous to the tail. It can be done in 3 steps:

  • Traverse the list and while traversing maintain the previous node address also. By the time we reach the end of the list, we will have two pointers, one pointing to the tail node and the other pointing to the node before the tail node.
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  • Update previous node’s next pointer with NULL.
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  • Dispose of the tail node.
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3. Deleting an Intermediate Node in Singly Linked List

In this case, the node to be removed is always located between two nodes. Head and tail links are not updated in this case. Such a removal can be done in two steps:

  • Similar to the previous case, maintain the previous node while traversing the list. Once we find the node to be deleted, change the previous node’s next pointer to the next pointer of the node to be deleted.
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  • Dispose of the current node to be deleted.
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Time complexity: O (n). In the worst we may need to delete the node at the end of the list.

Space Complexity: O (1). Since we are creating only one temporary variable. 

Single Linked List
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