1. Divide and
Conquer

2. Divide and Conquer
• divide the problem into a number of
subproblems
• conquer the subproblems (solve them)
• combine the subproblem solutions to get the
solution to the original problem
• Note: often the “conquer” step is done
recursively

3. Divide-and-Conquer
A general methodology for using
recursion to design efficient algorithms
It solves a problem by:
– Diving the data into parts
– Finding sub solutions for each of the parts
– Constructing the final answer from the sub
solutions

4. Divide and Conquer
• Based on dividing problem into
subproblems
• Approach
1. Divide problem into smaller subproblems
Subproblems must be of same type
Subproblems do not need to overlap
2. Solve each subproblem recursively
3. Combine solutions to solve original problem
• Usually contains two or more recursive
calls

5. Divide-and-conquer technique
a problem of size n
subproblem 1 subproblem 2
of size n/2 of size n/2
a solution to a solution to
subproblem 1 subproblem 2
a solution to
the original problem

6. Divide and Conquer Algorithms
• Based on dividing problem into subproblems
– Divide problem into sub-problems
Subproblems must be of same type
Subproblems do not need to overlap
– Conquer by solving sub-problems recursively. If
the sub-problems are small enough, solve them
in brute force fashion
– Combine the solutions of sub-problems into a
solution of the original problem (tricky part)

7. D-A-C
• For Divide-and-Conquer algorithms the
running time is mainly affected by 3
criteria:
• The number of sub-instances into which
a problem is split.
• The ratio of initial problem size to sub-
problem size.
• The number of steps required to divide
the initial instance and to combine sub-
solutions.

8. Algorithm for General Divide and Conquer
Sorting
• Algorithm for General Divide and Conquer
Sorting
• Begin Algorithm
Start Sort(L)
If L has length greater than 1 then
Begin
Partition the list into two lists, high and low
Start Sort(high)
Start Sort(low)
Combine high and low
End
• End Algorithm

9. Analyzing Divide-and-Conquer
Algorithms
• When an algorithm contains a recursive call
to itself, its running time can often be describ
ed by a recurrence equation which describes
the overall running time on a problem of size
n in terms of the running time on smaller inp
uts.
• For divide-and-conquer algorithms, we get
recurrences that look like:
•
• T(n) {
=
Θ(1)
aT(n/b) +D(n) +C(n)
if n < c

10. Analyzing Divide-and-Conquer Algorithms
(cont.)
• where
• a = the number of subproblems we break
the problem into
• n/b = the size of the subproblems (in
terms of n)
• D(n) is the time to divide the problem of
size n into the subproblems
• C(n) is the time to combine the
subproblem solutions to get the answer for
the problem of size n

11. The algorithm
• Lets assume the following array
2 6 7 3 5 6 9 2 4 1
• We divide the values into pairs
2 6 7 3 5 6 9 2 4 1
• We sort each pair
2 6 3 7 5 6 2 9 1 4
• Get the first pair (both lowest values!)

12. The algorithm (2)
• We compare these values (2 and 6) with the
values of the next pair (3 and 7)
2 6 3 7 5 6 2 9 1 4
– Lowest 2,3
• The next one (5 and 6)
– Lowest 2,3
• The next one (2 and 9)
– Lowest 2,2
• The next one (1 and 4)
– Lowest 1,2

13. Example: Divide and Conquer
• Binary Search
• Heap Construction
• Tower of Hanoi
• Exponentiation
– Fibonnacci Sequence
• Quick Sort
• Merge Sort
• Multiplying large Integers
• Matrix Multiplications
• Closest Pairs

14. Quicksort

15. Design
 Follows the divide-and-conquer paradigm.
 Divide: Partition (separate) the array A[p..r] into two
(possibly nonempty) subarrays A[p..q–1] and A[q+1..r].
 Each element in A[p..q–1] ≤ A[q].
 A[q] ≤ each element in A[q+1..r].
 Index q is computed as part of the partitioning
procedure.
 Conquer: Sort the two subarrays A[p..q–1] &
A[q+1..r] by recursive calls to quicksort.
 Combine: Since the subarrays are sorted in place –
no work is needed to combine them.
 How do the divide and combine steps of quicksort
compare with those of merge sort?

16. Pseudocode
Quicksort(A, p, r)
Quicksort(A, p, r) Partition(A, p, r)
Partition(A, p, r)
if pp< rrthen
if < then x:= A[r],
x:= A[r],
qq:= Partition(A, p, r);
:= Partition(A, p, r); i i:=p – 1;
:=p – 1;
Quicksort(A, p, qq––1);
Quicksort(A, p, 1); for jj:= ppto rr––11do
for := to do
Quicksort(A, qq+ 1, r)
Quicksort(A, + 1, r) if A[j] ≤ xxthen
if A[j] ≤ then
fi
fi ii:= ii+ 1;
:= + 1;
A[i] ↔ A[j]
A[i] ↔ A[j]
A[p..r] fi
fi
od;
od;
5 A[i + 1] ↔ A[r];
A[i + 1] ↔ A[r];
return ii+ 11
return +
A[p..q – 1] A[q+1..r]
Partition 5
≤5 ≥5

17. Example
p r
initially: 2 5 8 3 9 4 1 7 10 6 note: pivot (x) = 6
i j
next iteration: 2 5 8 3 9 4 1 7 10 6
i j Partition(A, p, r)
Partition(A, p, r)
x, ii := A[r], pp––1;
x, := A[r], 1;
next iteration: 2 5 8 3 9 4 1 7 10 6 for jj:= ppto rr––11do
for := to do
i j if A[j] ≤ xxthen
if A[j] ≤ then
ii:= ii+ 1;
:= + 1;
next iteration: 2 5 8 3 9 4 1 7 10 6 A[i] ↔ A[j]
A[i] ↔ A[j]
i j fi
fi
od;
od;
next iteration: 2 5 3 8 9 4 1 7 10 6
A[i + 1] ↔ A[r];
A[i + 1] ↔ A[r];
i j
return ii+ 1
return + 1

18. Example (Continued)
next iteration: 2 5 3 8 9 4 1 7 10 6
i j
next iteration: 2 5 3 8 9 4 1 7 10 6
i j
next iteration: 2 5 3 4 9 8 1 7 10 6
Partition(A, p, r)
Partition(A, p, r)
i j
x, ii := A[r], pp––1;
x, := A[r], 1;
next iteration: 2 5 3 4 1 8 9 7 10 6 for jj:= ppto rr––11do
for := to do
i j if A[j] ≤ xxthen
if A[j] ≤ then
ii:= ii+ 1;
:= + 1;
next iteration: 2 5 3 4 1 8 9 7 10 6 A[i] ↔ A[j]
A[i] ↔ A[j]
i j fi
fi
od;
od;
next iteration: 2 5 3 4 1 8 9 7 10 6
A[i + 1] ↔ A[r];
A[i + 1] ↔ A[r];
i j
return ii+ 1
return + 1
after final swap: 2 5 3 4 1 6 9 7 10 8
i j

19. Partitioning
 Select the last element A[r] in the subarray A[p..r] as
the pivot – the element around which to partition.
 As the procedure executes, the array is partitioned
into four (possibly empty) regions.
1. A[p..i] — All entries in this region are ≤ pivot.
2. A[i+1..j – 1] — All entries in this region are > pivot.
3. A[r] = pivot.
4. A[j..r – 1] — Not known how they compare to pivot.
 The above hold before each iteration of the for loop,
and constitute a loop invariant. (4 is not part of the LI.)

20. Correctness of Partition
Use loop invariant.
Initialization:
– Before first iteration
• A[p..i] and A[i+1..j – 1] are empty – Conds. 1 and 2 are
satisfied (trivially). Partition(A, p, r)
Partition(A, p, r)
x, i := A[r], p – 1;
• r is the index of the pivot – Cond. 3 is forij := p to r p – 1;
x, := A[r],
satisfied. – 1 do
for j := p to r – 1 do
Maintenance: if A[j] ≤ xxthen
if A[j] ≤ then
ii:= ii+ 1;
:= + 1;
– Case 1: A[j] > x A[i] ↔ A[j]
A[i] ↔ A[j]
• Increment j only. fi
fi
od;
od;
• LI is maintained. A[i + 1] ↔ A[r];
A[i + 1] ↔ A[r];
return ii+ 11
return +

21. Correctness of Partition
Case 1:
p i j r
>x x
≤x >x
p i j r
x
≤x >x

22. Correctness of Partition
• Case 2: A[j] ≤ x
– Increment i – A[r] is unaltered.
– Swap A[i] and A[j] • Condition 3 is maintained.
• Condition 1 is maintained.
– Increment j
• Condition 2 is maintained.
p i j r
≤x x
≤x >x
p i j r
x
≤x >x

23. Correctness of Partition
 Termination:
– When the loop terminates, j = r, so all elements in A are
partitioned into one of the three cases:
• A[p..i] ≤ pivot
• A[i+1..j – 1] > pivot
• A[r] = pivot
 The last two lines swap A[i+1] and A[r].
– Pivot moves from the end of the array to between the
two subarrays.
– Thus, procedure partition correctly performs the divide
step.

24. Complexity of Partition
• PartitionTime(n) is given by the number of
iterations in the for loop.
∀ Θ(n) : n = r – p + 1. Partition(A, p, r)
Partition(A, p, r)
x, ii := A[r], pp––1;
x, := A[r], 1;
for jj:= ppto rr––11do
for := to do
if A[j] ≤ xxthen
if A[j] ≤ then
ii:= ii+ 1;
:= + 1;
A[i] ↔ A[j]
A[i] ↔ A[j]
fi
fi
od;
od;
A[i + 1] ↔ A[r];
A[i + 1] ↔ A[r];
return ii+ 1
return + 1

25. Algorithm Performance
• Running time of quicksort depends on whether the
partitioning is balanced or not.
• Worst-Case Partitioning (Unbalanced Partitions):
– Occurs when every call to partition results in the most
unbalanced partition.
– Partition is most unbalanced when
• Subproblem 1 is of size n – 1, and subproblem 2 is of size 0
or vice versa.
• pivot ≥ every element in A[p..r – 1] or pivot < every element in
A[p..r – 1].
– Every call to partition is most unbalanced when
• Array A[1..n] is sorted or reverse sorted!

26. Worst-case Partition Analysis
Recursion tree for
worst-case partition
n
n–1 • Running time for worst-case
partitions at each recursive level:
• T(n) = T(n – 1) + T(0) +
n–2 PartitionTime(n)
n • = T(n – 1) + Θ(n)
n–3 • = ∑k=1 to nΘ(k)
• = Θ(∑k=1 to n k )
2 • = Θ(n2)
•
1

27. Best-case Partitioning
• Size of each subproblem ≤ n/2.
– One of the subproblems is of size n/2
– The other is of size n/2 −1.
• Recurrence for running time
– T(n) ≤ 2T(n/2) + PartitionTime(n)
= 2T(n/2) + Θ(n)
• T(n) = Θ(n lg n)

28. Recursion Tree for Best-case
Partition
cn cn
cn/2 cn/2 cn
lg n
cn/4 cn/4 cn/4 cn/4 cn
c c c c c c cn
Total : O(n lg n)

29. Conclusion
• • Divide and conquer is just one of several
• powerful techniques for algorithm design.
• • Divide-and-conquer algorithms can be
analyzed using recurrences and the
master method (so practice this math).
• • Can lead to more efficient algorithms

30. Divide and Conquer (Merge Sort)
Divide and Conquer (Merge Sort)

31. Divide and Conquer
• Recursive in structure
– Divide the problem into sub-problems
that are similar to the original but smaller
in size
– Conquer the sub-problems by solving
them recursively. If they are small
enough, just solve them in a
straightforward manner.
– Combine the solutions to create a
solution to the original problem

32. An Example: Merge Sort
• Sorting Problem: Sort a sequence of n
elements into non-decreasing order.
• Divide: Divide the n-element sequence to
be sorted into two subsequences of n/2
elements each
• Conquer: Sort the two subsequences
recursively using merge sort.
• Combine: Merge the two sorted
subsequences to produce the sorted

33. Merge Sort – Example
Original Sequence Sorted Sequence
18 26 32 6 43 15 9 1 1 6 9 15 18 26 32 43
18 26 32 6 43 15 9 1 6 18 26 32 1 9 15 43
43
18 26 32 6 43 15 9 1 18 26 6 32 15 43 1 9
18 26 32 6 43 15 9 1 18 26 32 6 43 15 9 1
18 26 32 6 43 15 9 1

34. Merge-Sort (A, p, r)
• INPUT: a sequence of n numbers stored in
array A
MergeSort (A, p, r) // sort A[p..r] by divide & conquer
• if p < r
1 OUTPUT: an ordered sequence of n
2 numbers (p+r)/2
then q ←
3 MergeSort (A, p, q)
4 MergeSort (A, q+1, r)
5 Merge (A, p, q, r) // merges A[p..q] with A[q+1..r]
Initial Call: MergeSort(A, 1, n)

35. Procedure Merge
• Merge(A, p, q, r)
• 1 n1 ← q – p + 1 Input: Array containing
• 2 n2 ← r – q sorted subarrays A[p..q]
 for i ← 1 to n1 and A[q+1..r].
 do L[i] ← A[p + i – 1] Output: Merged sorted
 for j ← 1 to n2
subarray in A[p..r].
 do R[j] ← A[q + j]
 L[n1+1] ← ∞
 R[n2+1] ← ∞
 i←1
 j←1 Sentinels, to avoid having to
 for k ←p to r check if either subarray is
 do if L[i] ≤ R[j] fully copied at each step.
 then A[k] ← L[i]
 i←i+1
 else A[k] ← R[j]
 j←j+1

36. Merge – Example
A … 6
1 8 26 32 26 32 42 43
6 8 9 1 9 …
k k k k k k k k k
L 6 8 26 32 ∞ R 1 9 42 43 ∞
i i i i i j j j j j

37. Correctness of Merge
• Merge(A, p, q, r)
Loop Invariant for the for loop
• 1 n1 ← q – p + 1 At the start of each iteration of the
• 2 n2 ← r – q for loop:
 for i ← 1 to n1 Subarray A[p..k – 1]
 do L[i] ← A[p + i – 1] contains the k – p smallest elements
 for j ← 1 to n2 of L and R in sorted order.
L[i] and R[j] are the smallest elements of
 do R[j] ← A[q + j] L and R that have not been copied back into
 L[n1+1] ← ∞ A.
 R[n2+1] ← ∞
 i←1 Initialization:
 j←1 Before the first iteration:
•A[p..k – 1] is empty.
 for k ←p to r
•i = j = 1.
 do if L[i] ≤ R[j]
•L[1] and R[1] are the smallest
 then A[k] ← L[i] elements of L and R not copied to A.
 i←i+1
 else A[k] ← R[j]

38. Correctness of Merge
• Merge(A, p, q, r) Maintenance:
• 1 n1 ← q – p + 1 Case 1: L[i] ≤ R[j]
•By LI, A contains p – k smallest elements
• 2 n2 ← r – q
of L and R in sorted order.
 for i ← 1 to n1 •By LI, L[i] and R[j] are the smallest elements
 do L[i] ← A[p + i – 1] of L and R not yet copied into A.
 for j ← 1 to n2 •Line 13 results in A containing p – k + 1
smallest elements (again in sorted order).
 do R[j] ← A[q + j] Incrementing i and k reestablishes the LI for
 L[n1+1] ← ∞ the next iteration.
 R[n2+1] ← ∞ Similarly for L[i] > R[j].
 i←1 Termination:
 j←1 •On termination, k = r + 1.
 for k ←p to r •By LI, A contains r – p + 1 smallest
 do if L[i] ≤ R[j] elements of L and R in sorted order.
 then A[k] ← L[i] •L and R together contain r – p + 3 elements.
All but the two sentinels have been copied
 i←i+1
back into A.
 else A[k] ← R[j]


39. Analysis of Merge Sort
• Running time T(n) of Merge Sort:
• Divide: computing the middle takes Θ(1)
• Conquer: solving 2 subproblems takes 2T(n/2)
• Combine: merging n elements takes Θ(n)
• Total:
T(n) = Θ(1) if n = 1
T(n) = 2T(n/2) + Θ(n) if n > 1
⇒ T(n) = Θ(n lg n) (CLRS, Chapter 4)