Linear Structures and Hash Tables
A dynamic set S usually supports the following "dictionary operations":
For a linear data structure, the order among elements is implicitly represented in an array, and explicitly in a linked list. Between the two, an array is more efficient in access at any position ("random access"), while a linked list is more efficient in insertions and deletions at a given position.
The elements of a linear structure can be data structures, such as in a matrix. In an Object-Oriented Programming language, linked lists are often implemented in two ways, with or without a specific "head" object containing the reference to the first node. Without such a head, the "node" class and the "list" class are merged, and the list is defined recursively as either empty or a node succeeded by a list.
A linked list can also be implemented in two arrays, one of which keeps the elements themselves, and the other keeps the index of the successor for each element. Similarly, multiple arrays can be used to represent an array of objects with multiple fields.
To turn a linear structure into a circular structure, in array A index = index mod A.length + 1 will do it, and in linked list we can let the last element point to the first element.
To move in both directions in a linked list, an additional link can be used to turn a singly-linked list into a doubly-linked list.List operrations:
Insert/Delete on stack and queue are taken as Θ(1) operations, and the size of the data structure automatically grows when needed.
Both stack and queue can be implemented by array and linked list:
|operations happen at the highest index value
|a circular array is needed to work on both ends
|a reference points to the last node
|two references points to both end, and in a singly-linked list, deletion can only happen at one end
Among the three operations, each insertion and deletion usually requires a search (as duplicate keys are not allowed), therefore search is the representative operation in efficiency analysis. Since search is normally a mapping from a key to an index in the table, the intuition behind hash table is to directly build such a mapping index = h(key), without comparing the key with the elements in the table. Since the range of index is much smaller than the range of key, the mapping is many-to-one, not one-to-one.
The design of a hash table consists of two major decisions: (1) to define a hash function index = h(key), (2) to handle "collision", the situation where multiple keys are mapped into the same index.
Typically, the size of a hash table is proportional to the number of keys actually stored, while being much smaller than the range of possible keys.
A good hash function satisfies (approximately) the assumption that each key is equally likely to hash to any of the m index values, independently of where any other key has hashed to. Often, a hash function works in two steps: (1) to convert a key into an integer, (2) to calculate the index from the integer. Therefore, hash function discussion often assumes that the function is a mapping from an integer (key) to an integer (index).
The most common hash function is to take the reminder of the key divided by the size of the hash table, that is, h(k) = k mod m, assuming index in [0, m − 1]. A more complicated version is h(k) = f(k) mod m, where f(k) does additional calculation to reduce collision, and the reminder operation makes the function value to cover the whole table. A similar approach is h(k) = floor(g(k) * m), where g(k) maps k into the range of [0, 1).
Common collision handling methods can be divided into two types, open addressing, where all elements are stored in the table itself, and separate chaining, where elements are stored outside the table in (sorted) linked-lists ("buckets"). Separate chaining requires additional space, though is conceptually simpler than open addressing.In open addressing, the hash function generates a probe sequence to tell the element where to go if the slot indicated by the hash function is already occupied by a different key. Such a sequence can depend on the key (such as double hashing) or follow a fixed pattern (such as linear probing and quadratic probing). In either way, element comparisons are necessary, as collision can happen in multiple places. After deletion, the released space is often marked for the following search to pass through. Since insertion includes search, a new key can be stored in a previously occupied space only after the searching stops at a never-occupied space somewhere down the path. Another strategy is lazy deletion that relocates some element in a following search. To make things simple, we can also only insert into an empty cell, and let the "previous-occupied" marks be cleared periodically by re-hashing all elements.
Under the assumption that each possible probing sequence is equally probable, and that the load factor of the hash table α (number of elements / table size) is less than 1, there are the following conclusions for open addressing: