added skip lists

[MrWeast]

added skip lists and a skip list implementation in python

# Description

skip lists are another common data structure and adding it will also allow for more opportunities for others to contribute in different languages

• added skip lists to read me
• added skip list implementation in python

skip_list.py

• skip connections made at random
• sorted order
• removing a node leaves a pseudo dead node
• automatically rebuilds with only alive nodes once more than half of the nodes are dead
• can be rebuilt whenever desired as well
• does not allow for duplicate numbers

methods

• reset: reinitialize values of Skip_List, used for rebuilding
• status:
  • reports the total number of unique elements
  • the number of which that are dead
  • the number of which are alive
• insert:
  • inserts a new element if the value is not already in list
  • randomly adds skip connections on new node
  • updates list head if a skip connection is added that is taller than the current height of the list
• find:
  • if the element is alive
    • returns the node with the corresponding value at the highest layer
  • otherwise returns None
• remove:
  • calls find
  • if node was found then sets it and duplicate skip connection nodes as dead
• show_layers:
  • will print out all nodes in each layer
  • can specify if you want to print only living nodes, only dead nodes, or both
• rebuild:
  - finds all alive nodes in Skip_List
  - calls reset
  - inserts all of the found alive nodes into Skip_List

code verification

the skip_list.py includes a main function which includes trying out different test cases such as inserting a new minimum value, trying to insert a duplicate, removing elements, causing a rebuild and so on.

It also includes inserting 50 arbitrary numbers, then removing 30 of them randomly, and then trying to remove 10 numbers that were never inserted at all.