#### Recursive Mergesort Array

I
```"""A merge sort which accepts an array as input and recursively
splits an array in half and sorts and combines them.
"""

"""https://en.wikipedia.org/wiki/Merge_sort """

def merge(arr: list[int]) -> list[int]:
"""Return a sorted array.
>>> merge([10,9,8,7,6,5,4,3,2,1])
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> merge([1,2,3,4,5,6,7,8,9,10])
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> merge([10,22,1,2,3,9,15,23])
[1, 2, 3, 9, 10, 15, 22, 23]
>>> merge([100])
[100]
>>> merge([])
[]
"""
if len(arr) > 1:
middle_length = len(arr) // 2  # Finds the middle of the array
left_array = arr[
:middle_length
]  # Creates an array of the elements in the first half.
right_array = arr[
middle_length:
]  # Creates an array of the elements in the second half.
left_size = len(left_array)
right_size = len(right_array)
merge(left_array)  # Starts sorting the left.
merge(right_array)  # Starts sorting the right
left_index = 0  # Left Counter
right_index = 0  # Right Counter
index = 0  # Position Counter
while (
left_index < left_size and right_index < right_size
):  # Runs until the lowers size of the left and right are sorted.
if left_array[left_index] < right_array[right_index]:
arr[index] = left_array[left_index]
left_index += 1
else:
arr[index] = right_array[right_index]
right_index += 1
index += 1
while (
left_index < left_size
):  # Adds the left over elements in the left half of the array
arr[index] = left_array[left_index]
left_index += 1
index += 1
while (
right_index < right_size
):  # Adds the left over elements in the right half of the array
arr[index] = right_array[right_index]
right_index += 1
index += 1
return arr

if __name__ == "__main__":
import doctest

doctest.testmod()
```