Lecture No. 11€¦ · View the entire tree with four subtrees connected with 3 nodes. k1 k2 X Y Z 4 Cases for Rotation Y is non-empty because the new node was inserted in Y. Thus

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Lecture No. 11

2

Cases for Rotation

Single right rotation fails to fix case 2.

k1

k2

Z

X

Level n

Level n-1

Level n-2

k1

k2

Z

X

new

Y

new

Y

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3

Cases for Rotation

Y is non-empty because the new node was inserted in Y.

Thus Y has a root and two subtrees.

View the entire tree with four subtrees connected with 3 nodes.

k1

k2

X

Y

Z

4

Cases for Rotation

Y is non-empty because the new node was inserted in Y.

Thus Y has a root and two subtrees.

View the entire tree with four subtrees connected with 3 nodes.

k1

k3

D

A

B C

k2

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5

Cases for Rotation

k3

k1

A

new

1

2

New node inserted a either of the two spots

B C

k2

D

new’

Exactly one of tree B or Cis two levels deeper than D; we are not sure which one.

Good thing: it does not matter.

To rebalance, k3 cannot be left as the root.

6

Cases for Rotation

k3

k1

A

new

1

2


B C

k2

D

new’

A rotation between k3

and k1 (k3 was k2 then) was shown to not work.

The only alternative is to place k2 as the new root.

This forces k1 to be k2‘s left child and k3 to be its right child.

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7

Cases for Rotation

Left-right double rotation to fix case 2.


A

new

1

2

B C

D

new’

k3

k1

k2

A

new

B

D

C

new’

k3

k2

k1

Rotate left

8

Cases for Rotation

Left-right double rotation to fix case 2.

A

new

B

D

C

new’A

new

B

D

C

new’

k3

k2

k1

Rotate right k2

k3k1

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9

Cases for Rotation

Right-left double rotation to fix case 3.

k1

k3

D

A

B C

k2

k1

k3

D

A

B

C

k2

Rotate right

10

Cases for Rotation

Right-left double rotation to fix case 3.

k1

k2

DAB C

k3

k1

k3

D

A

B

C

k2

Rotate left

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11

AVL Tree Building Example

Insert(15)

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5

6

X(null)

Z(null)Y

15

k1

k2

7

16

12


Insert(15) right-left double rotation

1

2

3

4

5

6

7

16

15

k1

k3

k2

Rotate right

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13



1

2

3

4

5

6

7

16

15

k1

k3

k2Rotate left

14



1

2

3

4

5

6

16

k3

15

k2

7

k1

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15


Insert(14): right-left double rotation

1

2

3

4

5

6

16

k3

15

k2

7

k1

14

Rotate right

16



1

2

3

4

5

6

16

k315

k27

k1

14

Rotate left

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17



1

2

3

4

5 16

k3

15

k2

7

6

k1

14

18


Insert(13): single rotation

1

2

3

4

5 16

15

7

6

14

13

Rotate left

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Insert(13): single rotation

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2

3

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5

16

15

7

6 14

13

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C++ code for insert

TreeNode<int>*

avlInsert(TreeNode<int>* root, int info)

{

if( info < root->getInfo() ){

root->se tLeft(avlInsert(root->getLeft(), info));

int htdiff = height(root->getLeft()) –height(root->getRight());

if( htdiff == 2 )

if( info < root->getLeft()->getInfo() )

root = singleRightRotation( root );

else

root = doubleLeftRightRotation( root );

}

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C++ code for insert

TreeNode<int>*


{


root->setLeft(avlInsert(root->getLeft(), info));


if( htdiff == 2 )



else


}

22

C++ code for insert

TreeNode<int>*


{




if( htdiff == 2 )



else


}

Root of left subtree may change

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C++ code for insert

TreeNode<int>*


{




if( htdiff == 2 )



else


}

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C++ code for insert

TreeNode<int>*


{




if( htdiff == 2 )



else


}

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C++ code for insert

TreeNode<int>*


{




if( htdiff == 2 )



else


}

Outside case

inside case

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C++ code for insert

else if(info > root->getInfo() ) {

root->setRight(avlInsert(root->getRight(),info));

int htdiff = height(root->getRight()) –height(root->getLeft());

if( htdiff == 2 )

if( info > root->getRight()->getInfo() )

root = singleLeftRotation( root );

else

root = doubleRightLeftRotation( root );

}

// else a node with info is already in the tree. In// case, reset the height of this root node.

int ht = Max(height(root->getLeft()),

height(root->getRight()));

root->setHeight( ht+1 ); // new height for root.

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27

C++ code for insert




if( htdiff == 2 )



else


}





28

C++ code for insert




if( htdiff == 2 )



else


}





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C++ code for insert




if( htdiff == 2 )



else


}





30

C++ code for insert




if( htdiff == 2 )



else


}





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C++ code for insert




if( htdiff == 2 )



else


}





32

C++ code for insert




if( htdiff == 2 )



else


}





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TreeNode<int>* singleRightRotation(TreeNode<int>* k2)

{

if( k2 == NULL ) return NULL;

// k1 (first node in k2's left subtree) // will be the new root

TreeNode<int>* k1 = k2->getLeft();

// Y moves from k1's right to k2's left

k2->setLeft( k1->getRight() );

k1->setRight(k2);

// reassign heights. First k2

int h = Max(height(k2->getLeft()),height(k2->getRight()));

k2->setHeight( h+1 );

// k2 is now k1's right subtree

h = Max( height(k1->getLeft()), k2->getHeight());


k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

34


{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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35


{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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37


{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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{






k1->setRight(k2);







k1

k2

Z

Y

X

k1

k2

ZYX

singleRightRotation

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int height( TreeNode<int>* node )

{

if( node != NULL ) return node->getHeight();

return -1;

}

height

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int height( TreeNode<int>* node )

{

if( node != NULL ) return node->getHeight();

return -1;

}

height

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TreeNode<int>* singleLeftRotation( TreeNode<int>* k1 )

{


// k2 is now the new root

TreeNode<int>* k2 = k1->getRight();

k1->setRight( k2->getLeft() ); // Y

k2->setLeft( k1 );

// reassign heights. First k1 (demoted)

int h = Max(height(k1->getLeft()),

height(k1->getRight()));


// k1 is now k2's left subtree

h = Max( height(k2->getRight()),

k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

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43


{





k2->setLeft( k1 );







k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

44


{





k2->setLeft( k1 );







k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

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45


{





k2->setLeft( k1 );







k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

46


{





k2->setLeft( k1 );







k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

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47


{





k2->setLeft( k1 );







k1->getHeight());


k1

k2

X

Y

Z

k1

k2

X YZ

singleLeftRotation

48

TreeNode<int>* doubleRightLeftRotation(TreeNode<int>* k1)

{


// single right rotate with k3 (k1's right child)

k1->setRight( singleRightRotation(k1->getRight()));

// now single left rotate with k1 as the root

return singleLeftRotation(k1);

} k1

k3

D

A

B C

k2

k1

k3

D

A

B

C

k2

doubleRightLeftRotation

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49


{






} k1

k3

D

A

B C

k2

k1

k3

D

A

B

C

k2


50


{






}

k1

k2

DAB C

k3

k1

k3

D

A

B

C

k2


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TreeNode<int>* doubleLeftRightRotation(TreeNode<int>* k3)

{


// single left rotate with k1 (k3's left child)

k3->setLeft( singleLeftRotation(k3->getLeft()));

// now single right rotate with k3 as the root

return singleRightRotation(k3);

}

k1

k3

D

A

B C

k2 k1

k3

D

A B

C

k2


52


{






}

k1

k3

D

A

B C

k2 k1

k3

D

A B

C

k2


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{






}

k1 k3

DAB C

k2

k1

k3

D

A B

C

k2


54

Deletion in AVL Tree

Delete is the inverse of insert: given a value X and an AVL tree T, delete the node containing X and rebalance the tree, if necessary.

Turns out that deletion of a node is considerably more complex than insert

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Insertion in a height-balanced tree requires at most one single rotation or one double rotation.

We can use rotations to restore the balance when we do a deletion.

We may have to do a rotation at every level of the tree: log2N rotations in the worst case.

56


Here is a tree that causes this worse case number of rotations when we delete A. At every node in N’s left subtree, the left subtree is one shorter than the right subtree.

A

C

D

N

E J

G

I

F

H

K

L

M

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57


Deleting A upsets balance at C. When rotate (D up, C down) to fix this

A

C

D

N

E J

G

I

F

H

K

L

M

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Deleting A upsets balance at C. When rotate (D up, C down) to fix this

C

D

N

E J

G

I

F

H

K

L

M

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59


The whole of F’s left subtree gets shorter. We fix this by rotation about F-I: F down, I up.

C

D

N

E

J

G

I

F

H

K

L

M

60


The whole of F’s left subtree gets shorter. We fix this by rotation about F-I: F down, I up.

C

D

N

E

J

G

I

F

H

K

L

M

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61


This could cause imbalance at N.

The rotations propagated to the root.

C

D

N

E

JG

I

F

H

K

L

M

62


Procedure

Delete the node as in binary search tree (BST).

The node deleted will be either a leaf or have just one subtree.

Since this is an AVL tree, if the deleted node has one subtree, that subtree contains only one node (why?)

Traverse up the tree from the deleted node checking the balance of each node.

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There are 5 cases to consider.

Let us go through the cases graphically and determine what action to take.

We will not develop the C++ code for deleteNode in AVL tree. This will be left as an exercise.

64


Case 1a: the parent of the deleted node had a balance of 0 and the node was deleted in the parent’s left subtree.

Action: change the balance of the parent node and stop. No further effect on balance of any higher node.

Delete on this side

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65


Here is why; the height of left tree does not change.

1

2

3

4

5

6

7

0

1

2

66


Here is why; the height of left tree does not change.

1

2

3

4

5

6

7

2

3

4

5

6

7

0

1

2

remove(1)

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67


Case 1b: the parent of the deleted node had a balance of 0 and the node was deleted in the parent’s right subtree.

Action: (same as 1a) change the balance of the parent node and stop. No further effect on balance of any higher node.

Delete on this side

68


Case 2a: the parent of the deleted node had a balance of 1 and the node was deleted in the parent’s left subtree.

Action: change the balance of the parent node. May have caused imbalance in higher nodes so continue up the tree.

Delete on this side

Documents

Lecture No. 11€¦ · View the entire tree with four subtrees connected with 3 nodes. k1 k2 X Y Z 4 Cases for Rotation Y is non-empty because the new node was inserted in Y. Thus