avl trees - cs.toronto.edutabrown/csc263/2014w/week3.pdf · avl trees. today •avl delete and...
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AVL trees
Today
• AVL delete and subsequent rotations
• Testing your knowledge with interactive
demos!
AVL tree
• Is a binary search tree
• Has an additional height constraint:
– For each node x in the tree, Height(x.left) differs
from Height(x.right) by at most 1
• I promise:
– If you satisfy the height constraint, then the
height of the tree is O(lg n).
– (Proof is easy, but no time! =])
AVL tree
• To be an AVL tree, must always:
– (1) Be a binary search tree
– (2) Satisfy the height constraint
• Suppose we start with an AVL tree, then delete as if we’re in a regular BST.
• Will the tree be an AVL tree after the delete?
– (1) It will still be a BST…
– (2) Will it satisfy the height constraint?
• (Not covering insert, since you already did in class)
BST Delete breaks an AVL tree
7
4
3
9
7
4
3
Delete(9)
h(left) > h(right)+1
so NOT an AVL tree!
Replacing the height constraint
with balance factors
• Instead of thinking about the heights of nodes, it
is helpful to think in terms of balance factors
• The balance factor bf(x) = h(x.right) – h(x.left)
– bf(x) values -1, 0, and 1 are allowed
– If bf(x) < -1 or bf(x) > 1 then tree is NOT AVL
Same example with bf(x), not h(x)
7
4
3
9
7
4
3
Delete(9)
-1
-1
0
0
-2
-1
0
bf < -1
so NOT an AVL tree!
What else can BST Delete break?
• Balance factors of ancestors…
7
4
3
9
7
4Delete(3)
-1
-1
0
0
-1
-1 900
0
Need a new Delete algorithm
• Goal: if tree is AVL before Delete, then tree is AVL after Delete.
• Step 1: do BST delete.
– This maintains the BST property, but cancause the balance factors of ancestors to be outdated!
• Step 2: fix the height constraint and update balance factors.
– Update any invalid balance factors affected by delete.
– After updating them, they can be < -1 or > 1.
– Do rotations to fix any balance factors that are too small or large while maintaining the BST property.
– Rotations can cause balance factors to be outdated also!
Bad balance factors
• Start with an AVL tree, then do a BST Delete.
• What bad values can bf(x) take on?
– Delete can reduce a subtree’s height by 1.
– So, it might increase or decrease h(x.right) –
h(x.left) by 1.
– So, bf(x) might increase or decrease by 1.
– This means:
• if bf(x) = 1 before Delete, it might become 2. BAD.
• If bf(x) = -1 before Delete, it might become -2. BAD.
• If bf(x) = 0 before Delete, then it is still -1, 0 or 1. OK.
2 cases
Problematic cases for Delete(a)
• bf(x) = -2 is just symmetric to bf(x) = 2.
• So, we just look at bf(x) = 2.
x2
h+2
h
x-2
a a(deleted)
Delete(a): 3 subcases for bf(x)=2
• Since tree was AVL before, bf(z) = -1, 0 or 1
Case bf(z) = -1 Case bf(z) = 0 Case bf(z) = 1
h+1
x
T2
T1
z
2
0
T3
ah
x
T2
T1
z
2
1
T3
a
x
T2
T1
z
2
h+1
-1h
a
T3
Fixing case bf(x) = 2, bf(z) = 0
• We do a single left rotation
• Preserves the BST property, and fixes bf(x) = 2
x
T2
T1
z
2
h+1
h0
T3
z
T2
T1
x
-1
1
T3
Fixing case bf(x) = 2, bf(z) = 1
• We do a single left rotation (same as last case)
• Preserves the BST property, and fixes bf(x) = 2
x
T2
T1
z
2
h+1
h1
T3
z
T2T1
x
0
0
T3
h
Delete(a): bf(x)=2, bf(z)=-1 subcases
Case bf(z) = -1: we have 3 subcases. (More details)
Case bf(y) = 0 Case bf(y) = -1 Case bf(y) = 1
x
T1
z
2
h
h-1
T3
a y
T21 T22
h-1
0
x
T1
z
2
-1
a y
T21
22T22h
-1
x
T1
z
2
-1
a y
21T21
T22
1
T3 T3
Double right-left rotation
• All three subcases of bf(x)=2, bf(z)=-1 simply
perform a double right-left rotation.
x
z
y
y
zx
x
y
z
Delete subcases for bf(x)=2, bf(z)=-1
• Case bf(y)=0: double right-left rotation!
x
T1
z
2
h
h-1
T3
y
T21 T22
0
h
y
T1
x
0
0
T3
z
T21T22
0
Delete subcases for bf(x)=2, bf(z)=-1
• Case bf(y)=-1: double right-left rotation!
x
T1
z
2
h
h-1
T3
y
T21
-1
h
y
T1
x
0
1
T3
z
T21
22T22
0
22T22
h-1
Delete subcases for bf(x)=2, bf(z)=-1
• Case bf(y)=1: double right-left rotation!
x
T1
z
2
h
h-1
T3
y
T22
1
h
y
T1
x
0
0
T3
z
21T21
T22
-1
21T21
h-1
Recursively fixing balance factors
• Idea: start at the node we deleted, fix a
problem, then recurse up the tree to the root.
• At each node x, we update the balance factor:
bf(x) := h(bf.right) - h(bf.left).
• If bf(x) = -2 or +2, we perform a rotation.
• Then, we update the balance factors of every
node that was changed by the rotation.
• Finally, we recurse one node higher up.
Interactive AVL Deletes
• Interactive web applet
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