2-3-4 Tree

src: https://sedgewick.io/talks/#ll-red-black-trees
Used in:Red Black Tree Left Leaning Red Black Tree

Definition

• Node
  • 2-node = 1 key 2 children
  • 3-node = 2 key 3 children
  • 4-node = 3 key 4 children
• Child
  • Left child = less than key.0
  • Middle.0 child = between key.0 and key.1
  • Middle.1 child = between key.1 and key.2
  • Right child = greater than key.2

Operation

• Search

  • 

• Insert

  1. Search to bottom for key
  2. If its a 2-node: convert it to 3-node
  3. If its a 3-node: convert it to 4-node.
  4. If its a 4-node:
     1. move the middle key to parent
     2. split remainder to two 2-node
     3. goto: 2
     • Problem here, parent may be 4-node
     • Solution 1: Use the same method to split parent, then continue up the tree if necessary. (Bottom up, like RBTree-FixUp)
     • Solution 2: Split 4-nodes on the way down, insert at bottom. (Top down)
  • Splitting 4-nodes on the way down
    • Put the middle to the up right
    • 4-node below a 4-node case never happens
    • Bottom node reached is always a 2-node or a 3-node
  5. 4-node below 2-node
     • 
  6. 4-node below a 3-node
     • 

Conclusion

Too complex for implementation
Enhancement: Left Leaning Red Black Tree