Principle and implementation of hash table, unordered set and mapping

Hashtable (Hash table) Also known as hash table , It's a way to get through key Data structure for direct access .

The hash table consists of two parts

• A data structure , Usually linked list 、 Array
• Hash function , Input key Return the index of the data structure

Example ：

hash_table["lies"]=233 Example , Egos ASCII Code addition mod20 by Hash function

 

If anything else key after hash Function calculation is also placed in 9 The location of , Then it will happen hash Collision .

hash Collision refers to two different key Be calculated the same Hash result , Map complex information to small value ranges , Collision is inevitable , well Hash Function can reduce the probability of collision , Let the data be distributed as evenly as possible

Hashing is the most common collision solution

• Hash The function is still used to calculate the array subscript
• Each position of the array stores a header pointer of the linked list （ We call it the header array ）
• Each linked list has the same Hash It's worth the data

  Time complexity

expect ： Insert 、 Inquire about 、 Delete O(1) --------- When the data distribution is relatively uniform

The worst ： Insert 、 Inquire about 、 Delete O(N)----------- The data is all mapped to the same Hash When the value of

Set and map

aggregate (set) Store non repeating elements

• Ordered set , Sort by element size when convenient , Generally, balanced binary search tree is used to realize ,O(logN)
• unordered set , It's usually used hash Realization ,O(1)

mapping (map) Store keys (key) Non repeating key value pairs (key-value pair)

• Ordered mapping , When traversing, follow key Size sorting , Generally, balanced binary search tree is used to realize ,O(logN)
• Unordered mapping , Generally, hash table is used to realize ,O(1)

For language built-in types （int,string）, There are already default excellent hash function , You can put it directly into set/map Use in