merge_sort.py

# Merge Sort implementation in Python with in depth comments
#
# Author: BedirT

# Merge Sort is a divide and conquer algorithm that divides the input array into two halves, 
# calls itself for the two halves, and then merges the two sorted halves. The merge() 
# function is used for merging two halves. The merge(arr, l, m, r) is key process that 
# assumes that arr[l..m] and arr[m+1..r] are sorted and merges the two sorted sub-arrays into one.

# Time Complexity: O(nlogn)
# Space Complexity: O(n)

def merge_sort(arr):
    if len(arr) > 1:
        # Finding the mid of the array
        mid = len(arr) // 2

        # Dividing the array elements
        L = arr[:mid]

        # into 2 halves
        R = arr[mid:]

        # Sorting the first half
        merge_sort(L)

        # Sorting the second half
        merge_sort(R)

        i = j = k = 0

        # Copy data to temp arrays L[] and R[]
        while i < len(L) and j < len(R):
            if L[i] < R[j]:
                arr[k] = L[i]
                i += 1
            else:
                arr[k] = R[j]
                j += 1
            k += 1

        # Checking if any element was left
        while i < len(L):
            arr[k] = L[i]
            i += 1
            k += 1

        while j < len(R):
            arr[k] = R[j]
            j += 1
            k += 1


# Code to print the list
def print_list(arr):
    for i in range(len(arr)):
        print(arr[i], end=" ")
    print()


# Driver Code
if __name__ == '__main__':
    arr = [12, 11, 13, 5, 6, 7]
    print("Given array is", end="\n")
    print_list(arr)
    merge_sort(arr)
    print("Sorted array is: ", end="\n")
    print_list(arr)