# Python Merge Sort Program

## Python Merge Sort

In this tutorial, we have implemented Merge Sort Algorithm. Also, by default, the merge_sort() function in the following program sorts the list in ascending order.

To get the descending order, all you have to do is just reverse the list.

Python Program

``````def merge_sort(nlist, start, end):
#sorts the list from indexes start to end - 1 inclusive
if end - start > 1:
mid = (start + end)//2
merge_sort(nlist, start, mid)
merge_sort(nlist, mid, end)
merge_list(nlist, start, mid, end)

def merge_list(nlist, start, mid, end):
left = nlist[start:mid]
right = nlist[mid:end]
k = start
i = 0
j = 0
while (start + i < mid and mid + j < end):
if (left[i] <= right[j]):
nlist[k] = left[i]
i = i + 1
else:
nlist[k] = right[j]
j = j + 1
k = k + 1
if start + i < mid:
while k < end:
nlist[k] = left[i]
i = i + 1
k = k + 1
else:
while k < end:
nlist[k] = right[j]
j = j + 1
k = k + 1

# Input list
aList = [1, 74, 96, 5, 42, 63]

merge_sort(aList, 0, len(aList))
print('Sorted List after Merge Sort, in Ascending Order\n', aList)

aList.reverse()
print('Sorted List after Merge Sort, in Descending Order\n', aList)``````
Run Code Copy

Output

``````Sorted List after Merge Sort, in Ascending Order
[1, 5, 42, 63, 74, 96]
Sorted List after Merge Sort, in Descending Order
[96, 74, 63, 42, 5, 1]``````

## Conclusion

In this tutorial of Python Examples, we learned how to implement Merge Sort Algorithm in Python.

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