singlyLinkedList.txt

LINKED LISTS:

Like arrays, Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at contiguous location; the elements are linked using pointers.
linkedlist

Why Linked List?

Arrays can be used to store linear data of similar types, but arrays have following limitations.
1) The size of the arrays is fixed: So we must know the upper limit on the number of elements in advance. Also, generally, the allocated memory is equal to the upper limit irrespective of the usage.
2) Inserting a new element in an array of elements is expensive, because room has to be created for the new elements and to create room existing elements have to shifted.

For example, in a system if we maintain a sorted list of IDs in an array id[].

id[] = [1000, 1010, 1050, 2000, 2040].
And if we want to insert a new ID 1005, then to maintain the sorted order, we have to move all the elements after 1000 (excluding 1000).
Deletion is also expensive with arrays until unless some special techniques are used. For example, to delete 1010 in id[], everything after 1010 has to be moved.

Advantages over arrays:

1) Dynamic size
2) Ease of insertion/deletion

Drawbacks:

1) Random access is not allowed. We have to access elements sequentially starting from the first node. So we cannot do binary search with linked lists efficiently with its default implementation. Read about it here.
2) Extra memory space for a pointer is required with each element of the list.
3) Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.

Representation:

A linked list is represented by a pointer to the first node of the linked list. The first node is called head. If the linked list is empty, then value of head is NULL.
Each node in a list consists of at least two parts:
1) data
2) Pointer (Or Reference) to the next node
In C, we can represent a node using structures. Below is an example of a linked list node with an integer data.
In Java, LinkedList can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

// A linked list node 
struct Node 
{ 
  int data; 
  struct Node *next; 
}; 

// A simple C program to introduce 
// a linked list 
#include<stdio.h> 
#include<stdlib.h> 
  
struct Node  
{ 
  int data; 
  struct Node *next; 
}; 
  
// Program to create a simple linked list with 3 nodes 
int main() 
{ 
  struct Node* head = NULL; 
  struct Node* second = NULL; 
  struct Node* third = NULL; 
    
  // allocate 3 nodes in the heap   
  head = (struct Node*)malloc(sizeof(struct Node));  
  second = (struct Node*)malloc(sizeof(struct Node)); 
  third = (struct Node*)malloc(sizeof(struct Node)); 
  
  /* Three blocks have been allocated  dynamically.  
     We have pointers to these three blocks as first, 
     second and third      
       head           second           third 
        |                |               | 
        |                |               | 
    +---+-----+     +----+----+     +----+----+ 
    | #  | #  |     | #  | #  |     |  # |  # | 
    +---+-----+     +----+----+     +----+----+ 
     
   # represents any random value. 
   Data is random because we haven’t assigned  
   anything yet  */
    
  head->data = 1; //assign data in first node 
  head->next = second; // Link first node with the second node 
    
  /* data has been assigned to data part of first 
     block (block pointed by head).  And next 
     pointer of first block points to second.   
     So they both are linked. 
  
       head          second         third 
        |              |              | 
        |              |              | 
    +---+---+     +----+----+     +-----+----+ 
    | 1  | o----->| #  | #  |     |  #  | #  | 
    +---+---+     +----+----+     +-----+----+     
  */  
    
  // assign data to second node  
  second->data = 2;  
  
  // Link second node with the third node 
  second->next = third; 
    
  /* data has been assigned to data part of second 
     block (block pointed by second). And next 
     pointer of the second block points to third  
     block. So all three blocks are linked. 
    
       head         second         third 
        |             |             | 
        |             |             | 
    +---+---+     +---+---+     +----+----+ 
    | 1  | o----->| 2 | o-----> |  # |  # | 
    +---+---+     +---+---+     +----+----+      */    
    
  third->data = 3; //assign data to third node 
  third->next = NULL; 
    
  /* data has been assigned to data part of third 
    block (block pointed by third). And next pointer 
    of the third block is made NULL to indicate 
    that the linked list is terminated here. 
  
     We have the linked list ready.   
  
           head     
             | 
             |  
        +---+---+     +---+---+       +----+------+ 
        | 1  | o----->|  2  | o-----> |  3 | NULL | 
        +---+---+     +---+---+       +----+------+        
     
      
    Note that only head is sufficient to represent  
    the whole list.  We can traverse the complete  
    list by following next pointers.    */      
  
  return 0; 
}