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May 17, 2012 18:16
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Recursive Mergesort in Python
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def merge(left, right): | |
if not len(left) or not len(right): | |
return left or right | |
result = [] | |
i, j = 0, 0 | |
while (len(result) < len(left) + len(right)): | |
if left[i] < right[j]: | |
result.append(left[i]) | |
i+= 1 | |
else: | |
result.append(right[j]) | |
j+= 1 | |
if i == len(left) or j == len(right): | |
result.extend(left[i:] or right[j:]) | |
break | |
return result | |
def mergesort(list): | |
if len(list) < 2: | |
return list | |
middle = len(list)/2 | |
left = mergesort(list[:middle]) | |
right = mergesort(list[middle:]) | |
return merge(left, right) | |
if __name__ == "__main__": | |
print mergesort([3,4,5,1,2,8,3,7,6]) |
Thank you for this piece of code)
By the way, if someone find it interesting, here is merge realisation with deque from python collections:
from collections import deque
from itertools import islice
def merge(a, b):
c = deque([])
while a and b:
i = a[0]
j = b[0]
if i <= j:
c.append(i)
a.popleft()
else:
c.append(j)
b.popleft()
if a:
c.extend(a)
if b:
c.extend(b)
return c
def merge_sort(A):
if len(A) < 2:
return A
m = len(A) // 2
l = merge_sort(deque(islice(A, 0, m)))
r = merge_sort(deque(islice(A, m, len(A))))
return merge(l, r)
if __name__ == '__main__':
print(merge_sort([3, 4, 5, 1, 2, 8, 3, 7, 6]))
Here is merge sort in one function with detailed information on running process
def merge_sort(nlist):
print("Splitting ", nlist)
if len(nlist) > 1:
mid = len(nlist) // 2
lefthalf = nlist[:mid]
righthalf = nlist[mid:]
merge_sort(lefthalf)
merge_sort(righthalf)
i = j = k = 0
while i < len(lefthalf) and j < len(righthalf):
if lefthalf[i] < righthalf[j]:
nlist[k] = lefthalf[i]
i = i + 1
else:
nlist[k] = righthalf[j]
j = j + 1
k = k + 1
while i < len(lefthalf):
nlist[k] = lefthalf[i]
i = i + 1
k = k + 1
while j < len(righthalf):
nlist[k] = righthalf[j]
j = j + 1
k = k + 1
print("Merging ", nlist)
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Nice and Simple . I made a small change on the same idea and got rid of the breaks .
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