下列Python归并排序算法程序中，横线处应该填入的是（）
def merge_sort(array):
if len(array) == 1:
return array
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return merge(left, right)
def merge(left, right):
left_index, right_index, merge_array = 0, 0, list()
while left_index < len(left) and right_index < len(right):
if left[left_index] <= right[right_index]:
merge_array.append(left[left_index])
left_index += 1
else:
merge_array.append(right[right_index])
right_index += 1
merge_array = merge_array + left[left_index:] + right[right_index:]
return merge_array