Special-Purpose Data Structures
Special-Purpose Data Structures
The special-purpose data structures discussed in this book are the stack, the queue, and the priority queue. These structures, rather than supporting a database of user- accessible data, are usually used by a computer program to aid in carrying out some algorithm. We’ve seen examples of this throughout this book, such as in Chapters
13, “Graphs,” and 14, “Weighted Graphs,” where stacks, queues, and priority queues are all used in graph algorithms.
Stacks, queues, and priority queues are Abstract Data Types (ADTs) that are imple- mented by a more fundamental structure such as an array, linked list, or (in the case of the priority queue) a heap. These ADTs present a simple interface to the user, typi- cally allowing only insertion and the ability to access or delete only one data item. These items are
• For stacks: the last item inserted • For queues: the first item inserted • For priority queues: the item with the highest priority
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These ADTs can be seen as conceptual aids. Their functionality could be obtained using the underlying structure (such as an array) directly, but the reduced interface they offer simplifies many problems.
These ADTs can’t be conveniently searched for an item by key value or traversed.
Stack
A stack is used when you want access only to the last data item inserted; it’s a Last- In-First-Out (LIFO) structure.
A stack is often implemented as an array or a linked list. The array implementation is efficient because the most recently inserted item is placed at the end of the array, where it’s also easy to delete. Stack overflow can occur, but is not likely if the array is reasonably sized, because stacks seldom contain huge amounts of data.
If the stack will contain a lot of data and the amount can’t be predicted accurately in advance (as when recursion is implemented as a stack), a linked list is a better choice than an array. A linked list is efficient because items can be inserted and deleted quickly from the head of the list. Stack overflow can’t occur (unless the entire memory is full). A linked list is slightly slower than an array because memory alloca- tion is necessary to create a new link for insertion, and deallocation of the link is necessary at some point following removal of an item from the list.
Queue
A queue is used when you want access only to the first data item inserted; it’s a First-In-First-Out (FIFO) structure.
Like stacks, queues can be implemented as arrays or linked lists. Both are efficient. The array requires additional programming to handle the situation in which the queue wraps around at the end of the array. A linked list must be double-ended, to allow insertions at one end and deletions at the other.
As with stacks, the choice between an array implementation and a linked list imple- mentation is determined by how well the amount of data can be predicted. Use the array if you know about how much data there will be; otherwise, use a linked list.
Priority Queue
A priority queue is used when the only access desired is to the data item with the highest priority. This is the item with the largest (or sometimes the smallest) key.
Priority queues can be implemented as an ordered array or as a heap. Insertion into an ordered array is slow, but deletion is fast. With the heap implementation, both insertion and deletion take O(logN) time.
724 CHAPTER 15 When to Use What
Use an array or a double-ended linked list if insertion speed is not a problem. The array works when the amount of data to be stored can be predicted in advance; the linked list when the amount of data is unknown. If speed is important, a heap is a better choice.