分为3个函数实现：
sort()：主排序函数
_sort()：分治函数，将排序数组不断分解为子数组进行排序
_merge()：归并函数，将两个有序数组合并成一个大的有序数组
1 def sort(a): 2 n = len(a) 3 aux = [0]*n 4 return _sort(a,0,n,aux) 5 6 def _sort(a,lo,hi,aux): 7 n = hi - lo 8 if n <= 1:return 9 mid = (lo + hi) // 2 10 _sort(a,lo,mid,aux) 11 _sort(a,mid,hi,aux) 12 return _merge(a,lo,mid,hi,aux) 13 14 def _merge(a,lo,mid,hi,aux): 15 n = hi - lo 16 i = lo 17 j = mid 18 for k in range(n): 19 if i == mid:aux[k] = a[j];j += 1 20 elif j == hi:aux[k] = a[i];i += 1 21 elif a[j]<a[i]:aux[k] = a[j];j += 1 22 else:aux[k] = a[i];i += 1 23 a[lo:hi]=aux[0:n] 24 return a 25 26 >>x = [1,3,2,5,4] 27 >>print(sort(x))
注意：每次_sort()都是对左闭右开区间进行排序
另一种写法：
1 import random, time 2 3 def merge_sort(lst): 4 if len(lst) <= 1: 5 return lst # 从递归中返回长度为1的序列 6 7 middle = len(lst) / 2 8 left = merge_sort(lst[:middle]) # 通过不断递归，将原始序列拆分成n个小序列 9 right = merge_sort(lst[middle:]) 10 return merge(left, right) 11 12 def merge(left, right): 13 i, j = 0, 0 14 result = [] 15 while i < len(left) and j < len(right): # 比较传入的两个子序列，对两个子序列进行排序 16 if left[i] <= right[j]: 17 result.append(left[i]) 18 i += 1 19 else: 20 result.append(right[j]) 21 j += 1 22 result.extend(left[i:]) # 将排好序的子序列合并 23 result.extend(right[j:]) 24 return result 25 26 if __name__ == "__main__": 27 start = time.clock() 28 29 rand_lst = [] 30 for i in range(6): 31 rand_lst.append(round(random.random()*100, 2)) 32 lst = merge_sort(rand_lst) 33 34 end = time.clock() 35 print lst 36 print "done ", (end-start)