Question: 
Write a function to sort the given array by using merge sort.


Model's Code:
def merge_sort(arr):
    if len(arr) <= 1:
        return arr
    mid = len(arr) // 2
    left = merge_sort(arr[:mid])
    right = merge_sort(arr[mid:])
    return merge(left, right)

def merge(left, right):
    sorted_array = []
    left_index, right_index = 0, 0
    while left_index < len(left) and right_index < len(right):
        if left[left_index] < right[right_index]:
            sorted_array.append(left[left_index])
            left_index += 1
        else:
            sorted_array.append(right[right_index])
            right_index += 1
    sorted_array.extend(left[left_index:])
    sorted_array.extend(right[right_index:])
    return sorted_array



Sample Correct Code:
def merge(a,b):
    c = []
    while len(a) != 0 and len(b) != 0:
        if a[0] < b[0]:
            c.append(a[0])
            a.remove(a[0])
        else:
            c.append(b[0])
            b.remove(b[0])
    if len(a) == 0:
        c += b
    else:
        c += a
    return c
def merge_sort(x):
    if len(x) == 0 or len(x) == 1:
        return x
    else:
        middle = len(x)//2
        a = merge_sort(x[:middle])
        b = merge_sort(x[middle:])
        return merge(a,b)



Asserts:
assert merge_sort([3, 4, 2, 6, 5, 7, 1, 9]) == [1, 2, 3, 4, 5, 6, 7, 9]
assert merge_sort([7, 25, 45, 78, 11, 33, 19]) == [7, 11, 19, 25, 33, 45, 78]
assert merge_sort([3, 1, 4, 9, 8]) == [1, 3, 4, 8, 9]


Tests:
Test 1 of 3: PASS
Test 2 of 3: PASS
Test 3 of 3: PASS

Test 152 of 974: 110/152 (72.36842105263158%): success