LINKED LIST

In computer science, a linked list is one of the simplest data structures used in computer programming. It consists of a number of nodes, each containing one element and one (or more) links to other nodes. Linked lists permit insertion and removal of nodes at any point in the list in O(1) time. Linked lists do not allow random access.

The simplest kind of linked list is a singly linked list, which has one link per node. This link points to the next node in the list, or to a null value if it is the last node. This kind of list allows sequential access to elements only in one direction (from front to back).

A more sophisticated kind of linked list is a doubly linked list. Each node has two links, one to the previous node and one to the next. This allows sequential access to the list in both directions.

There are two significant variations to the types mentioned above. First is a circularly linked list, where the first and last nodes are linked together. This can be done for both singly and doubly linked lists. The other variation is the addition of a sentinel node at the beginning of the list. This node represents before the first node and/or after the last node of the list. It also makes it possible to have an empty list.

Singly linked lists have one strong advantage over doubly-linked lists: each tail of the list, meaning all the nodes following a chosen one, themselves form a singly linked list. Thus this type of list, sometimes also called a cons-based list, can be seen as a recursive datatype. This enables, for example, two different lists to share a common tail, which in turn enables one to add to the front of the list while keeping a reference to both the new and the old versions, making this type of list a persistent data structure.

Language Support

Many programming languages, such as LISP and Scheme have singly linked lists built in;. In other languages it is simple to create the data structure using pointers or references to implement links between list nodes. For example, in C:

typedef struct Person
{
    struct Person *prev; // previous person in list
    struct Person *next; // next person in list
    char name[10];
    int age;
};

Person first; // first item in the linked list
Person *p = &first;

while(!feof(stdin)) // there's still input
{
    p->next = (Person *)malloc(sizeof(Person));   // first link
    p = p->next;
    printf("Enter name and age:\
");              // Ask for input
    scanf("%s %d", &(p->name), &(p->age));        // Use it
}

Linked Lists On The Web

Some linked list materials are available from the Stanford Computer Science department:

• Introduction to Linked Lists
• Linked List Problems