Computer notes - singleRightRotation

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Class No.21

Data Structures

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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->setHeight( h+1 ); return k1;}

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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 )

{

if( k1 == NULL ) return NULL;

// 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->setHeight( h+1 );

// k1 is now k2's left subtree

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

k1->getHeight());


return k2;

}

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{





k2->setLeft( k1 );







k1->getHeight());


return k2;

}

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{





k2->setLeft( k1 );







k1->getHeight());


return k2;

}

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{





k2->setLeft( k1 );







k1->getHeight());


return k2;

}

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{





k2->setLeft( k1 );







k1->getHeight());


return k2;

}

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{





k2->setLeft( k1 );







k1->getHeight());


return k2;

}

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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);

}

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{






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{






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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);

}

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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.


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Deletion in AVL Tree 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.

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Deletion in AVL Tree Deleting A upsets balance at C. When rotate (D up, C down)

to fix this

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Deletion in AVL Tree Deleting A upsets balance at C. When rotate (D up, C down)

to fix this

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Deletion in AVL Tree The whole of F’s left subtree gets shorter. We fix this by

rotation about F-I: F down, I up.

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Deletion in AVL Tree The whole of F’s left subtree gets shorter. We fix this by

rotation about F-I: F down, I up.

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Deletion in AVL Tree This could cause imbalance at N. The rotations propagated to the root.

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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.


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

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

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remove(1)

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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

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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

Computer notes - singleRightRotation

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