Lecture 5_ Sorting and order statistics.pptx

Design and Analysis of
Algorithms
Master Theorem
Department of Computer Science

Asymptotic Behavior of Recursive
Algorithms
When analyzing algorithms, recall that we
only care about the asymptotic behavior
Recursive algorithms are no different
Rather than solving exactly the recurrence
relation associated with the cost of an
algorithm, it is sufficient to give an
asymptotic characterization
The main tool for doing this is the master
theorem

Master Theorem
Let T(n) be a monotonically increasing function that
satisfies
T(n) = a T(n/b) + f(n)
T(1) = c
where a  1, b  2, c>0. If f(n) is (nd) where d  0
then
If a < bd
T(n) = If a = bd
If a > bd

Master Theorem: Pitfalls
You cannot use the Master Theorem if
 T(n) is not monotone, e.g. T(n) = sin(x)
 f(n) is not a polynomial, e.g., T(n)=2T(n/2)+2n
 b cannot be expressed as a constant, e.g.
Note that the Master Theorem does not
solve the recurrence equation

Master Theorem: Example 1
Let T(n) = T(n/2) + ½ n2 + n. What are the
parameters?
a =
b =
d =
Therefore, which condition applies?
1
2
2
1 < 22, case 1 applies
• We conclude that
T(n)  (nd) =  (n2)

 Let T(n)= 2T(n/4) + n + 42.
 What are the parameters?
a =
b =
d =
2
4
1/2
2 = 41/2, case 2 applies

 Let T(n)= 3 T(n/2) + 3/4n + 1.
 What are the parameters?
a =
b =
d =
3
2
1
3 > 21, case 3 applies
• Note that log231.584…, can we say that T(n)   (n1.584)
No, because log231.5849… and n1.584   (n1.5849)

Exercises
For each recurrence, either give the asympotic
solution using the Master Theorem (state which
case), or else state that the Master Theorem
doesn't apply.
T(n) = 3T(n/2) + n2
T(n) = 7T(n/2) + n2
T(n) = 4T(n/2) + n2
T(n) = 3T(n/4) + n lg n
T(n) = T(n - 1) + n
T(n) = 2T(n/4) + n0.51

Design and Analysis of
Algorithms
Sorting and order statistics
Department of Computer Science

Heaps
A heap is a complete binary tree, where the
entry at each node is greater than or equal to
the entries in its children.
When a complete binary tree is built, its first
node must be the root.
The second node is always the left child of the
root
The third node is always the right child of the
root.
The nodes always fill each level from left-to-
right.

Definition
Max Heap
 Store data in ascending order
 Has property of
A[Parent(i)] ≥ A[i]
Min Heap
 Store data in descending order
 Has property of
A[Parent(i)] ≤ A[i]

Heaps
The heap property requires that
each node's key is >= to the
keys of its children.
The biggest node is always at the top in a Maxheap.
Therefore, a heap can implement a priority queue
(where we need quick access to the highest priority
item).
19
4
22
21
27
23
45
35

Max Heap Example
16
19 1 4
12 7
Array A
19
12 16
4
1 7

Min heap example
12
7 19
16
4
1
Array A
1
4 16
12
7 19

Heapify
 Heapify picks the largest child key and compares it to the parent
key. If parent key is larger then heapify quits, otherwise it swaps
the parent key with the largest child key. So that the parent is
larger than its children.
HEAPIFY (A,i)
p LEFT(i) -- assign left index to p
q RIGHT(i) -- assign right index to q
if p<=heap-size[A] and A[p] > A[i] -- check left child and if larger than parent assign
then largest  p -- largest= index of left child
else largest  i -- largest= index of parent
if q<=heap-size[A] and A[q] > A[largest] -- check right child and if larger than parent or left child
then largest  p -- largest= index of right child
if largest ≠ i -- only swap when parent is less than either child
then exchange A[i] with A[largest]
HEAPIFY (A,largest)

Analyzing Heapify(): Formal
Fixing up relationships between i, l, and r takes
(1) time
If the heap at i has n elements, how many
elements can the subtrees at l or r have?
Answer: 2n/3 (worst case: bottom row 1/2 full)
So time taken by Heapify() is given by
T(n)  T(2n/3) + (1)
By case 2 of the Master Theorem,
T(n) = O(lg n)
Thus, Heapify() takes logarithmic time

BUILD HEAP
 We can use the procedure 'Heapify' in a bottom-up fashion to convert
an array A[1 . . n] into a heap. Since the elements in the subarray
A[n/2 +1 . . n] are all leaves, the procedure BUILD_HEAP goes
through the remaining nodes of the tree and runs 'Heapify' on each
one. The bottom-up order of processing node guarantees that the
subtree rooted at the children are a heap before 'Heapify' is run at
their parent.
BUILD-HEAP (A)
Heap-size [A]  length [A]
for ilength[A/2] to 1
do HEAPIFY (A,i)
Running Time:
 Each call to HEAPIFY costs O(lg n)
 There are n calls: O(n)
O(n lg n)

Adding a Node to a Heap
Put the new node in the next available spot.
Push the new node upward, swapping with its
parent until the new node reaches an acceptable
location.
19
4
22
21
27
23
45
35
42

19
4
22
21
42
23
45
35
27
19
4
22
21
35
23
45
42
27

In general, there are two conditions that can stop
the pushing upward:
1. The parent has a key that is >= new node, or
2. The node reaches the root.
The process of pushing the new node upward is
called reheapification upward.

Removing the Top of a Heap
Move the last node
onto the root.
19
4
22
21
35
23
45
42
27

Removing the Top of a Heap
The process of pushing the new
node downward is called
reheapification downward.
19
4
22
21
27
23
42
35
Reheapification downward can stop under two
circumstances:
The children all have keys <= the out-of-place node, or
The node reaches the leaf.

Implementing a Heap
We will store the
data from the
nodes in a
partially-filled
array.
An array of data
21
27
23
42
35

Implementing a Heap
Data from the root goes in the first location of the
array.
An array of data
21
27
23
42
35
42

Implementing a Heap
Data from the next row
goes in the next two
array locations.
An array of data
21
27
23
42
35
42 35 23

Implementing a Heap
Data from the next row goes
in the next two array
locations.
An array of data
21
27
23
42
35
42 35 23 27 21

Heap Sort
 The heapsort algorithm consists of two phases:
- build a heap from an arbitrary array
- use the heap to sort the data
 To sort the elements in the decreasing order, use a min
heap
 To sort the elements in the increasing order, use a max heap
HEAPSORT(A)
BUILD-HEAP (A)
for i  length[A] down to 2
do exchange A[1] with A[i]
heap-size[A]  heap-size[A]-1 --discard node n from heap by
decrementing
HEAPIFY (A,1)

Heapsort
Given BuildHeap(), an in-place sorting
algorithm is easily constructed:
 Maximum element is at A[1]
 Discard by swapping with element at A[n]
 Decrement heap_size[A]
 A[n] now contains correct value
 Restore heap property at A[1] by calling
Heapify()
 Repeat, always swapping A[1] for
A[heap_size(A)]

Convert the array to a heap
16 4 7 1 12 19
Picture the array as a complete binary tree:
16
4 7
12
1 19

16
4 7
12
1 19
16
4 19
12
1 7
16
12 19
4
1 7
19
12 16
4
1 7
swap
swap
swap
Convert the array to a heap

Time Analysis of Heapsort
Build Heap Algorithm will run in O(n) time
There are n-1 calls to Heapify each call
requires O(log n) time
Heap sort program combine Build Heap
program and Heapify, therefore it has the
running time of O(n log n) time
Total time complexity: O(n log n)

Possible Application
When we want to know the task that carry the
highest priority given a large number of things to
do
Interval scheduling, when we have a list of
certain task with start and finish times and we
want to do as many tasks as possible
Sorting a list of elements that needs an efficient
sorting algorithm

Heapsort advantages
The primary advantage of heap sort is its
efficiency.
The execution time efficiency of heap sort is O(n
log n).
The memory efficiency of the heap sort, unlike
the other n log n sorts, is constant, O(1),
because heap sort algorithm is not recursive.

Priority Queues
A priority queue is a collection of zero or more elements
 each element has a priority or value
Unlike the FIFO queues, the order of deletion from a
priority queue (e.g., who gets served next) is determined
by the element priority
Elements are deleted by increasing or decreasing order
of priority rather than by the order in which they arrived
in the queue
Types of priority queue
Minimum priority queue: Returns the smallest element
and removes it from the data structure.
Maximum priority queue: Returns the largest element
and removes it from the data structure.

Priority queue methods
A priority queue supports the following methods:
 pop()/GetNext: ; Removes the item with the highest priority from
the queue.
 Push()/InsertWithPriority: add an element to the queue with an
associated priority.
 top()/PeekAtNext look at the element with highest priority
without removing it. This may be based on either the minimum or
maximum operation depending on the priority definition of the
priority queue
 Priority size(): Returns the number of elements in the priority
queue.
 empty(): Returns true if the priority queue is empty.

Priority Queue Operations
 Insert(S, x) inserts the element x into set S
 Maximum(S) returns the element of S with
the maximum key
 ExtractMax(S) removes and returns the
element of S with the maximum key

Advantages/Disadvantages
Advantages
 They enable you to retrieve items not by the insertion
time (as in a stack or queue).
 They offer flexibility in allowing client application
programs to perform a variety of different operations on
sets of records with keys. numerical keys (priorities)
Disadvantages
 Generally slow: Deletion of elements make a location
empty. The search of items include the empty spaces
too. This takes more time. To avoid this, use an empty
indicator or shift elements forward when each element is
deleted. We can also maintain priority queue as an array
of ordered elements, to avoid risk in searching

Implementation of priority queue
Binary heap: uses heap data structure-must be a
complete binary tree, first element is the root,
root>=child. Complexity- O(log n)- insertion and
deletion.
Binary search tree: may be a complete binary tree
or not, LeftChild<=root<=RightChild - Priority queue
is implemented using in order traversal on a BST.
Complexity- O(log n) - insertion, O(1) - deletion
Sorted array or list: elements are arranged linearly
based on priorities. Highest priority comes first.
Complexity- O(n) - insertion, O(1)- deletion.
Fibonacci heap: Complexity- O(log n).
Binomial heap: Complexity- O(log n).

Priority Queue Sorting
 We can use a priority
queue to sort a set of
comparable elements
1. Insert the elements one
by one with a series of
insert operations
2. Remove the elements in
sorted order with a
series of removeMin
operations
 The running time of this
sorting method depends on
the priority queue
implementation
Algorithm PQ-Sort(S, C)
Input sequence S, comparator C
for the elements of S
Output sequence S sorted in
increasing order according to C
P  priority queue with
comparator C
while S.isEmpty ()
e  S.removeFirst ()
P.insert (e, 0)
while P.isEmpty()
e  P.removeMin().key()
S.insertLast(e)

Sequence-based Priority Queue
 Implementation with an
unsorted list
 Performance:
 insert takes O(1) time
since we can insert the
item at the beginning or
end of the sequence
 removeMin and min take
O(n) time since we have
to traverse the entire
sequence to find the
smallest key
 Implementation with a
sorted list
 Performance:
 insert takes O(n) time
since we have to find the
place where to insert the
item
 removeMin and min take
O(1) time, since the
smallest key is at the
beginning
4 5 2 3 1 1 2 3 4 5

Insertion-Sort
 Insertion-sort is the variation of PQ-sort where the
priority queue is implemented with a sorted
sequence
 Running time of Insertion-sort:
1. Inserting the elements into the priority queue
with n insert operations takes time proportional
to 1 + 2 + …+ n
2. Removing the elements in sorted order from the
priority queue with a series of n removeMin
operations takes O(n) time
 Insertion-sort runs in O(n2) time

Insertion-Sort Example
Sequence S Priority queue P
Input: (7,4,8,2,5,3,9) ()
Phase 1
(a) (4,8,2,5,3,9) (7)
(b) (8,2,5,3,9) (4,7)
(c) (2,5,3,9) (4,7,8)
(d) (5,3,9) (2,4,7,8)
(e) (3,9) (2,4,5,7,8)
(f) (9) (2,3,4,5,7,8)
(g) () (2,3,4,5,7,8,9)
Phase 2
(a) (2) (3,4,5,7,8,9)
(b) (2,3) (4,5,7,8,9)
.. .. ..
. . .
(g) (2,3,4,5,7,8,9) ()

In-place Insertion-sort
 Instead of using an external
data structure, we can
implement insertion-sort in-
place
 A portion of the input
sequence itself serves as the
priority queue
 For in-place insertion-sort
 We keep sorted the initial
portion of the sequence
 We can use swaps instead of
modifying the sequence
5 4 2 3 1
5 4 2 3 1
4 5 2 3 1
2 4 5 3 1
2 3 4 5 1
1 2 3 4 5
1 2 3 4 5

Lecture 5_ Sorting and order statistics.pptx

More Related Content

Similar to Lecture 5_ Sorting and order statistics.pptx (20)

Recently uploaded (20)

Lecture 5_ Sorting and order statistics.pptx