Implementing Red Black Trees Ellen Walker CPSC 201 Data Structures Hiram College

Implementing Red-Black Trees Several implementations are possible (and many are on the Internet) –Some include parent links in RedBlackNodes - can be iterative –Textbook’s is recursive, recognizing need for rotations on the way back up the tree (after recursive call) These slides follow your textbook, but… –Code here is incomplete –In places, code has been simplified for presentation For complete, correct code, see your textbook and/or the book code

About These Slides Follow textbook’s presentation but… … code is incomplete … code is slightly simplified for ease of presentation –Example: casts are removed Use these notes as a guide for understanding when reading the actual textbook code

Classes (Koffman & Wolfgang) SearchTree BinarySearchTree BSTWithRotate RBTree (abstract class) (Adds RotateLeft and RotateRight)

Single Rotation (RotateRight) root temp = root.left; root.left = temp.right; temp.right = root; return temp; temp root

Double Rotation root root.left = rotateLeft(root.left); return RotateRight(root); 8

RedBlackNode E data –Data contained in this node RedBlackNode left, RedBlackNode right –References to children boolean isRed –Color of this node (true means Red) –Set to true in constructor, because all nodes (except root) are red when added

Recall: BST Recursive Add If (item.equals(localRoot.data)) addreturn = null; //duplicates not added Return localRoot; If (item.compareTo(localRoot.data)<0) Add to the left return localRoot; Else Add to the right return localRoot;

Recall: BST Recursive Add (left side) If localRoot.left == null localRoot.left = new Node(item); return localRoot; Else localRoot.left = add(localRoot.left,item); return localRoot;

Add Recursion Base case at leaf of tree –Create and link a new node Recursive case –Recursively add on the correct side –Link result of recursion back into the tree, or the changes will be lost! –localRoot.left = add(localRoot.left, item); //not just add(localRoot.left, item);

Top Down Insertion Algorithm Search the binary tree in the usual way for the insertion algorithm If you pass a 4-node (black node with two red children) on the way down, split it Insert the node as a red child, and use the split algorithm to adjust via rotation if the parent is red also. Force the root to be black after every insertion.

RBT Adding to the Left If localRoot.left == null localRoot.left = new Node(item); return localRoot; Else //Split me if I’m a 4-node moveBlackDown(localRoot); localRoot.left = add(localRoot.left, item); //Deal with consecutive red child & grandchild //case 1: red left child with red left grandchild //case 2: red left child with red right grandchild

RotateRight to Fix Left-Left Reds localRoot localRoot.left.isRed = false; localRoot.isRed = true; Return rotateRight(localRoot);

Double Rotation to Fix Left-Right Reds localRoot localRoot.left = rotateLeft(localRoot.left); localRoot.left.isRed = false; localRoot.isRed = true; return RotateRight(localRoot);

Putting the code all together Insert appropriate tests to recognize the various red-red cases Repeat all the “left code” for the right side –Swap “left” vs. “right” everywhere Write the “starter add method” –Deals with an empty tree –Makes the root black at the end

Starter Method If (root == null) { root = new RedBlackNode (item); root.isRed = false; //root is always black addReturn = true; //item was added } Else { root = add(root, item); //sets addReturn root.isRed = false; //root is always black } return addReturn;

Insert 1, 2, Insert red leaf (2 consecutive red nodes!) 2 31 Left single rotation

Continued: Insert 4, node (2 red children) split on the way down Root remains black Single rotation to avoid consecutive red nodes

Continued, Insert 9, node (3,4,5) split on the way down, 4 is now red (passed up) Double rotation

Deletion Algorithm Find the node to be deleted (NTBD) –On the way down, if you pass a 2-node upgrade it by borrowing from its neighbor and/or parent If the node is not a leaf node, –Find its immediate successor, upgrading all 2- nodes –Swap value of leaf node with value of NTBD Remove the current leaf node, which is now NTBD (because of swap, if it happened)

Red-black “neighbor” of a node Let X be a 2-node to be deleted If X is its parent’s left child, X’s right neighbor can be found by: –Let S = parent’s right child. If S is black, it is the neighbor –Otherwise, S’s left child is the neighbor. If X is parent’s left child, then X’s left neighbor is grandparent’s left child.

Neighbor examples Right neighbor of 1 is 3 Right neighbor of 3 is 5 Left neighbor of 5 is 3 Left neighbor of 3 is 1

Upgrade a 2-node Find the 2-node’s neighbor (right if any, otherwise left) If neighbor is also a 2-node (2 black children) –Create a 4-node from neighbors and their parent. –If neighbors are actually siblings, this is a color swap. –Otherwise, it requires a rotation If neighbor is a 3-node or 4-node (red child) –Move “inner value” from neighbor to parent, and “dividing value” from parent to 2-node. –This is a rotation

Deletion Examples (Delete 1) Make a 4-node from 1, sibling 3, and “divider value” 2. [Single rotation of 2,4,6] 1

Delete Upgrade 6 by color flip, swap with successor (9)

Delete No 2-nodes to upgrade, swap with successor (5)

Delete Find 2-node enroute to successor (3) Neighbor is 3-node (6,9) Shift to get (3,5) and 9 as children, 6 up to parent Single rotation

Delete 2 (cont’d) Swap with successor Remove leaf