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2
Regular Binary Trees
5
18
35
25
17 75
6
127
209
153
40
5
18
35
25
17
75
6 12
7
20
9
153
40
Depth = 6Average comparisons per search = 4.0
Depth = 4Average comparisons per search = 3.47
(a) (b)
100
100
Insertion sequence: 5, 15, 20, 3, 9, 7, 12, 17, 6, 75, 100, 18, 25, 35, 40
Insertion sequence: 9, 5, 20, 3, 7, 15, 75, 6, 12, 17, 35, 100, 18, 25, 40
3
Balanced TreesNeed a new search-tree structure - a balanced binary
search tree
1. Maintains balanced node when adding or removing them
Extra time needed at insert and remove2. Guarantees at worst O(log n) search time
We start with 2-3-4 trees…- Perfectly balanced- Difficult and inefficient to implement
4
2-3-4 Trees Definitions
2-node - a data value and pointers to 2 subtrees
3-node - two data values and pointers to 3 subtrees
4-node - three data values and pointers to 4 subtrees
A<B
A<B<C
5
2-3-4 Tree
2 15 35 55
12
254 8 10
115 7 9
2-3-4 trees have search tree propertiesWhat is the basic 2-3-4 search algorithm?
6
Inserting into a 2-3-4 Tree
A
A B C
S T VU
A
B
S T VUInsert begins with a single nodeand adds elements until it is full Insert another item
1. split the 4 node using the medianvalue as the parent (promoting the medianvalue to the parent level) 2. Insert the new item(inserted based on the BST rules for insertion)(always insert node in leaf of tree)
C
8
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 12 152 2 15
Insert 2 Insert 12Insert 15
9
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 12 152 2 15
Insert 2 Insert 12Insert 15
152
12
Split 4-node (2, 12, 15)
15
12
2 4
Insert 4
10
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 12 152 2 15
Insert 2 Insert 12Insert 15
152
12
Split 4-node (2, 12, 15)
15
12
2 4
Insert 4
15
12
2 4 8
11
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 12 152 2 15
Insert 2 Insert 12Insert 15
152
12
Split 4-node (2, 12, 15)
15
12
2 4
Insert 4
15
12
2 4 8
2
4 12
158 2
4 12
158 10
Split 4-node (2, 4, 8) Insert 10
12
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 12 152 2 15
Insert 2 Insert 12Insert 15
152
12
Split 4-node (2, 12, 15)
15
12
2 4
Insert 4
15
12
2 4 8
2
4 12
158 2
4 12
158 10
Split 4-node (2, 4, 8) Insert 10
2
4 12
8 10 15 25 2
4 12
2
4 12 25
8 10 15 35 2
4 12 25
8 10 15 35 55
15 25 358 10
Insert 25 Insert 35
Insert 55Split 4-node (15, 25, 35)
13
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 8 10 15 35 55
254
12
2 8 10 11 15 35 55
254
12
Split 4-node (15, 25, 35) Insert 11(4,12,25)
Proactive top-down approach to splitting a 4-node
14
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 8 10 15 35 55
254
12
2 8 10 11 15 35 55
254
12
Split 4-node (15, 25, 35) Insert 11
2 15 35 55
25
12
4 10
118 2 15 35 55
25
12
4 10
118 9
Split 4-node (8, 10, 11) Insert 9
(4,12,25)
15
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 15 35 55
25
12
4 10
115 8 9 2 15 35 55
25
12
4 8 10
1195
Insert 5 Split 4-node (5, 8, 9)
16
2-3-4 Insertion ExampleInsertion Sequence: 2, 15, 12, 4, 8, 10, 25, 35, 55, 11, 9, 5, 7
2 15 35 55
25
12
4 8 10
1195 7
2 15 35 55
25
12
4 10
115 8 9 2 15 35 55
25
12
4 8 10
1195
Insert 5 Split 4-node (5, 8, 9)
17
Another example
Keys: A S E R C H I N G X
What would the 2-3-4 tree look like after inserting this set of keys?
28
2-3-4 Tree Facts
• With N elements, the max number of nodes visited during the search for an element is int(log2 n) + 1
• Inserting an element into a tree with n elements requires splitting no more than int(log2 n) + 1 4-nodes (often far fewer)
• Problem: allocated a large amount of wasted space3n + 1 unused pointers (n - nodes)
2 15 35 55
25
12
4 8 10
1195 7
28 wasted pointers
29
2-3-4 Warmup Exercise
Create a 2-3-4 tree from the following sequence of numbers:
45 6 23 5 78 9 10 11 48 99 12 55
30
2-3-4 Warmup Exercise
Create a 2-3-4 tree from the following sequence of numbers:
45 6 23 5 78 9 10 11 48 99 12 55
31
Red-Black Trees• Designed to represent
2-3-4 tree without the additional link overhead.
• Colors are used to represent the 3-node and 4-node.
Red-Black trees are simple binary trees with a color indicator that is used to maintain certain properties - these properties keep the tree balanced.
32
Red-Black Nodes
2-nodes simple binary node
(black node)
4-nodes center value becomes the parent (black) with outside values becoming the children (red)
N
H I N
I
H N
33
Red-Black Nodes
3-nodes
A B
A
B
B
A
or
Note:1. Red-black trees are not
unique2. However, the
corresponding 2-3-4 tree is unique
34
Red-Black Nodes
A
B
B
A
N
I
H N
Use color grouping of nodes to indicate the corresponding nodes in the 2-3-4 tree
35
Converting a 2-3-4 Tree to Red-Black Tree Example
12 15
8 10 20
9 30 401 3 4
Top-down conversion algorithm: (start at the root)1. Apply red-black tree representation to each node2. Repeat for next level…
36
Converting a 2-3-4 Tree to Red-Black Tree Example
12 159 30 401 3 4
10
8 20
12 15
8 10 20
9 30 401 3 4
37
Converting a 2-3-4 Tree to Red-Black Tree Example
12 159 30 401 3 4
10
8 20
12 15
8 10 20
9 30 401 3 4
H I N I
H N
38
Converting a 2-3-4 Tree to Red-Black Tree Example
12 159 30 401 3 4
10
8 20
12 15 30 40
10
8 20
3 9
1 4
39
Converting a 2-3-4 Tree to Red-Black Tree Example
12 15 30 40
10
8 20
3 9
1 4
10
8 20
3 9
1 4 30
12 40
15
40
Converting a 2-3-4 Tree to Red-Black Tree Example
12 15 30 40
10
8 20
3 9
1 4
10
8 20
3 9
1 4 30
12 40
15
How could this be different?
41
Red-Black Tree Properties
1. The Root of a red-black tree is BLACK
2. A RED parent never has a RED child – there are never 2 RED nodes in succession
3. Every path from the root to an empty subtree (NULL pointer) has the same number of BLACK nodes.
a BLACK node corresponds to a level change in the 2-3-4 tree
10
8 20
3 9
1 4 30
12 40
15
12 15
8 10 20
9 30 401 3 4
42
Inserting Nodes Guidelines
1. Maintain root as BLACK node
2. Enter a new node as a RED node – since each new node enters a 2-node or a 3-node.
3. Whenever it results in two RED nodes in succession – rotate nodes to create a BLACK parent.
4. When scanning down a path to find insertion location – split any 4-node.
10
8 20
3 9
1 4 30
12 40
15
51
Splitting a 4-node
Parent P is BLACKX is a left-child
X
A B
P
C
Parent P is RED X is a right-child
Parent P is RED X is a left-child
X
A B
P
C
Parent P is BLACKX is a right-child
X
A B
P
C X
A B
P
C
4 possible situations
Step1: color flip – parent x becomes RED and its two children become BLACK.
52
Splitting a 4-nodeParent: BLACK
2-3-4 tree viewRed-black tree
before the color flip
A X B
P
C X P
BA
C
X
A B
P
C
Red-black treeafter the color flip 2-3-4 tree view
X
A B
P
C
Color flip is enough.
53
Splitting a 4-nodeand inserting node 55
2-3-4 tree view
50
40 60
30
2040 50 60 50
40 60
30
20
55
55 60
20 30 50
2-3-4 tree view
4-nodebefore color-flip
4-node after color-flipand insertion of 55
20 30
C D
C D C D
C D 40
Color flip is enough.
54
Splitting a 4-nodeParent: RED
Single right rotation withpivot P and color changes 2-3-4- tree view
2-3-4 tree view Red-black treebefore color flip
Red-black treeafter color flip
P G
A X B C D
G
BA
X
P
C
D
G
BA
X
P
C
D
G
BA
X
P
C D
X P G
A B C D
Splitting a 4-node oriented left-left from node G using a single right rotation
55
Splitting a 4-nodeParent: RED
Splitting a 4-node oriented left-right from node G after the color flip
P G
A X B
BA
X
G
P
C
DCD
BA
X
G
P
C
D
2-3-4 tree view Red-black treebefore color flip
Red-black treeafter color flip
56
Splitting a 4-nodeParent: RED
Splitting a 4-node oriented left-right from node G after the color flip
Red-Black Tree Property Violation?
P G
A X B
BA
X
G
P
C
DCD
BA
X
G
P
C
D
2-3-4 tree view Red-black treebefore color flip
Red-black treeafter color flip
57
Two REDs: rotations
X
GP
A B
A
P X G
BC
D
C
D
A
G
P
X
B
D
C
SingleLeft-rotationAbout X
P
G
X
A
B
D
C
SingleRight-rotationAbout X
59
Building a RED-BLACK tree
2, 15, 12, 4, 8, 10, 25, 35, 55,11, 9, 5, 7
Split whenGoing down?
55
11
63
Building a RED-BLACK tree
10, 20, 30, 40, 50, 31, 34, 36
No problem up to this point.What would the 2-3-4 tree look like?
64
Deleting RED-BLACK nodes
To Delete: same strategy as deleting any node from a binary tree (Chapter 10)
Delete node 25
Replace deleted node with value nearest to deleted value: either 15 or 30
65
Deleting RED-BLACK nodes
If replace node is RED then level height of tree is unchanged - no other action is needed except making sure color properties are maintained