歸併排序原理
採用分而治之的原理：
一、將乙個序列從中間位置分成兩個序列；
二、在將這兩個子串行按照第一步繼續二分下去；
三、直到所有子串行的長度都為1，也就是不可以再二分截止。這時候再兩兩合併成乙個有序序列即可。
一圖以明之
兩種實現方式：
一、
def merge(a, b):
c =
h = j = 0
while j < len(a) and h < len(b):
if a[j] < b[h]:
j += 1
else:
h += 1
if j == len(a):
for i in b[h:]:
else:
for i in a[j:]:
return c
def merge_sort(lists):
if len(lists) <= 1:
return lists
middle = len(lists)//2
left = merge_sort(lists[:middle])
right = merge_sort(lists[middle:])
return merge(left, right)
if __name__ == '__main__':
a = [14, 2, 34, 43, 21, 19]
print (merge_sort(a))
def mergesort(alist):
print("splitting ", alist)
if len(alist) > 1:
mid = len(alist) // 2
lefthalf = alist[:mid]
righthalf = alist[mid:]
mergesort(lefthalf)
mergesort(righthalf)
i = 0
j = 0
k = 0
print('alist',alist)
while i < len(lefthalf) and j < len(righthalf):
print('one',len(lefthalf),len(righthalf))
if lefthalf[i] < righthalf[j]:
alist[k] = lefthalf[i]
i = i + 1
print('one', alist)
else:
alist[k] = righthalf[j]
j = j + 1
print('one1', alist)
k = k + 1
while i < len(lefthalf):
print('two', len(lefthalf), len(righthalf))
alist[k] = lefthalf[i]
i = i + 1
k = k + 1
print('two', alist)
while j < len(righthalf):
print('three', len(lefthalf), len(righthalf))
alist[k] = righthalf[j]
j = j + 1
k = k + 1
print('three', alist)
print("merging ", alist)
alist = [54,26,93,17,77,31,44,55,20]
mergesort(alist)
print(alist)
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