归并排序 Python

将数组从中间分为两部分，一直分，一直分，分到只剩两两一堆，再全都组合回来。

def mergesort(arr):
	if nums == None or len(arr) < 2:
		return arr
	sortProcess(arr, 0, len(arr) - 1)
	return arr

def sortProcess(arr, l, r):
	if l == r:
		return arr
	mid = (l + r) // 2
	sortProcess(arr, l, mid)
	sortProcess(arr, mid + 1, r)
	merge(arr, l, mid, r)
def merge(arr, l, mid, r):
	helpd = []
	i = l
	j = mid +1
	while (i <= mid and j <= r):
		if arr[i] < arr[j]:
			helpd.append(arr[i])
			i += 1
		else:
			helped.append(arr[j])
			j += 1
	if i <= mid:
		helpd += arr[i : mid+1]
	if j <= r:
		helpd += arr[j : r+1]
	arr[l : r + 1] = helpd[:]
nums = [9,8,2,6,4,3,5,1,7]
mergesort(nums)
print(nums)