In this slide I explained about merge sort algorithm. By reading attentively and watching my slide topic you will easily understand merge sort algorithm.

1. Welcome

2. Department: Computer Science &
Engineering
Course Title: Algorithm lab
Course code: CSE 222
Semester: 4th
Batch: 25th

3. Created by:
Md Abdul Kuddus
Email: ma.kuddus37@gmail.com
Linkedin profile: https://www.linkedin.com/in/kuddus137

4. Presentation Topic:
MERGE SORT

5. MERGE SORT :
Merge sort was invented by John Von Neumann
(1903 - 1957)
Merge sort is divide & conquer technique of sorting
element
Merge sort is one of the most efficient sorting
algorithm
Time complexity of merge sort is O(n log n)

6. DIVIDE & CONQUER
• DIVIDE : Divide the unsorted list into two sub lists of
about half the size
• CONQUER : Sort each of the two sub lists recursively.
If they are small enough just solve them in a straight
forward manner
• COMBINE : Merge the two-sorted sub lists back into
one sorted list

7. DIVIDE & CONQUER TECHNIQUE
a problem of size n
sub problem 1 of
size n/2
sub problem 2 of
size n/2
a solution to sub
problem 1
a solution to sub
problem 2
a solution to the
original problem

8. MERGE SORT PROCESS :
• The list is divided into two equal (as equal as
possible) part
• There are different ways to divide the list into two
equal part
• The following algorithm divides the list until the list
has just one item which are sorted
• Then with recursive merge function calls these
smaller pieces merge

9. MERGE SORT ALGORITHM :
• Merge(A[],p , q , r)
{ n1 = q – p + 1
n2 = r – q
Let L[1 to n1+1] and R[1 to n2+1] be new array
for(i = 1 to n1)
L[i] = A[p + i - 1]
for(j = 1 to n2)
R[j] = A[q + j]
L[n1 + 1] = infinity
R[n2 + 1] = infinity

10. for(k = p to r)
{ if ( L[i] <= R[j])
A[k] = L[j]
i = i + 1
else
A[k] = R[j]
j = j + 1
}

11. Merge sort (A, p, r)
{ if (p < r) // check for base case
q = [ (p + r)/2 ] // divide step
Merge sort (A, p, q) // conquer step
Merge sort (A, q+1, r) // conquer step
Merge sort (A, p, q, r) // conquer step
}

12. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1K
1 2 3 4 5 6 7 8

13. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2K

14. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4K

15. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4 5K

16. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4 5 6K

17. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4 5 6 7K

18. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4 5 6 7 8K

19. MERGE SORT EXAMPLE :
1 5 7 8 2 4 6 9
p q q + 1 r
1 5 7 8 infinity 2 4 6 9 infinityL R
i=1 2 3 4 5 j=1 2 3 4 5
1 2 4 5 6 7 8 9K

20. /* Merging (Merge sort) */
#include <stdio.h>
void main ()
{
int a[10] = {1, 5, 11, 30, 2, 4, 6, 9};
int i,j,k,n1,n2,p,q,r;
p = 1; q = 4; r = 8; n1 = q-p+1; n2 = r-q;
int L[n1+1];
int R[n2+1];
for( i=0; i<n1; i++)
L[i] = a[p+i-1];
for( i=0; i<n1; i++)
printf("%d n", L[i]);
for( j=0; j<n2; j++)
R[j] = a[q+j];
for ( j=0; j<n2; j++)
printf("%d n", R[j]);

21. L[n1] = 300; R[n2] = 300; i=0; j=0;
for( k=0; k<r; k++)
{
if (L[i] <= R[j])
{
a[k] = L[i];
i++;
}
else
{
a[k] = R[j];
j++;
}
}
printf("n");
for( k=0; k<r; k++)
printf("%d n", a[k]);
}

22. IMPLEMENTING MERGE SORT :
There are two basic ways to implement
merge sort
In place : Merge sort is done with only the
input array
Double storage : Merging is done with a
temporary array of the same size as the input
array

23. MERGE-SORT ANALYSIS :
Time, merging
log n levels
Total time for merging : O (n log n)
Total running time : Order of n log n
Total space : Order of n
n/2 n/2
n/4 n/4 n/4 n/4
n

24. Performance of MERGESORT :
• Unlike quick sort, merge sort guarantees O (n log n)in the
worst case
• The reason for this is that quick sort depends on the value
of the pivot whereas merge sort divides the list based on
the index
• Why is it O (n log n ) ?
• Each merge will require N comparisons
• Each time the list is halved
• So the standard divide & conquer recurrence applies to
merge sort
T(n) = 2T * n/2 + (n)

25. FEATURES OF MERGE SORT :
• It perform in O(n log n ) in the worst case
• It is stable
• It is quite independent of the way the initial list is organized
• Good for linked lists. Can be implemented in such a way that data is
accessed sequentially.
Drawbacks :
It may require an array of up to the size of the original list.
This can be avoided but the algorithm becomes significantly more
complicated making it worth while.
Instead of making it complicated we can use HEAP SORT which is also
O(n log n ). But you have to remember that HEAP SORT is not stable in
comparison to MERGE SORT

26. QUERIES
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THANK YOU