Separate Chaining

A scheme in which each position in the hash table has a list to handle collisions. Each position may be just a link to the list (direct chaining) or may be an item and a link, essentially, the head of a list. In the latter, one item is in the table, and other colliding items are in the list.
Also known as external chaining.

Separate chaining with list heads :
Some chaining implementations store the first record of each chain in the slot array itself. The purpose is to increase cache efficiency of hash table access. In order to save memory space, such hash tables are often designed to have about as many slots as stored entries, meaning that many slots will have two or more entries.

Separate chaining with other structures :
Instead of a list, one can use any other data structure that supports the required operations. By using a self-balancing tree, for example, the theoretical worst-case time of a hash table can be brought down to O(log n) rather than O(n). However, this approach is worth the trouble and extra memory cost only if long delays must be avoided at all costs or if one expects to have many entries hashed to the same slot.
The variant called array hashing uses a dynamic array to store all the entries that hash to the same bucket.Each inserted entry gets appended to the end of the dynamic array that is assigned to the hashed slot.