Red-black Tree
External
- https://www.coursera.org/learn/algorithms-graphs-data-structures/lecture/8acpe/red-black-trees
- https://www.youtube.com/watch?v=scfDOof9pww
- https://en.wikipedia.org/wiki/Red%E2%80%93black_tree
Internal
Overview
Red-black trees were invented by Bayer (1972) and Guibas, Sedgewick (1978). A red-black tree is a type of binary search tree that self-balances on insertion and deletion, thus maintaining its height to a minimum, which allows for efficient operations. Almost all binary search tree operations have a running time bounded by the tree height, and in this case the tree height stays constant at log n, yielding O(log n) operations.
Red-black Tree Invariants
- Each node is either red or black.
- The root is always black.
- There are never 2 red nodes in a row (a red node has only black children).
- Every path taken from root to a NULL pointer - unsuccessful search - has the same number of black nodes (red makes a node "invisible" to the invariant 4).
These invariants are in addition to the fact that the red-black tree is a binary search tree, so it has the Binary Search Tree Property.
Red-black Trees Maintain Small Heights
If all these invariants are satisfied at all times, the height of the tree is going to be small. For example, a chain with three nodes cannot be a red-black tree.
Claim: every red-black tree with n nodes has height ≤ 2 log(n+1) (O(log n)).
Proof. We will prove that a red-black tree must look like a perfectly balanced tree with at most factor 2 inflation.
Suppose there is a binary tree where if you start from the root and no matter how you navigate through the tree to a NULL pointer, you have no choice but to see at least k nodes along the way. If this hypothesis is satisfied, then the top of the tree has to be fully filled in. Why? What if for all paths but one I see k nodes, and for two paths I see k + 1? The proof is by contradiction, if you were missing some nodes in any of these top k levels, that would. give you a way of hitting a NULL pointer seeing less than k nodes. This observation gives a lower bound on the population of a search tree as a function of the lengths of its root - NULL paths. The size n of the tree must include at least the number of nodes in a perfectly balanced tree of depth k - 1, which is 2k-1-1. [...]
TODO: continue.
Supported Operations
INSERT
The general idea is to implement insertion as for an ordinary binary search tree INSERT() operation, and if we find that after the insertion one or more of the red-black tree invariants are broken, we fix them.
TODO
DELETE
TODO