Description of Linked Lists
Singly-linked lists, usually simply called linked lists, are composed of individual elements, each linked by a single pointer. Each element consists of two parts: a data member and a pointer, called the next pointer. Using this two-member structure, a linked list is formed by setting the next pointer of each element to point to the element that follows it (see Figure 5.1). The next pointer of the last element is set to NULL, a convenient sentinel marking the end of the list. The element at the start of the list is its head; the element at the end of the list is its tail.
To access an element in a linked list, we start at the head of the list and use the next pointers of successive elements to move from element to element until the desired element is reached. With singly-linked lists, the list can be traversed in only one direction—from head to tail—because each element contains no link to its predecessor. Therefore, if we start at the head and move to some element, and then wish to access an element preceding it, we must start over at the head (although sometimes we can anticipate the need to know an element and save a pointer to it). Often this weakness is not a concern. When it is, we use a doubly-linked list or circular list.
Conceptually, one thinks of a linked list as a series of contiguous elements. However, because these elements are allocated dynamically (using malloc in C), it is important to remember that, in actuality, they are usually scattered about in memory (see Figure 5.2). The pointers from element to element therefore are the only means by which we can ensure that all elements remain accessible. With this in mind, we will see later that special care is required when it comes to maintaining the links. If we mistakenly drop one link, it becomes impossible to access any of the elements from that point on in the list. Thus, the expression “You are only as strong as your weakest link” is particularly fitting for linked lists.
