我正在阅读我的算法书（Intro to算法第3版Cormen）中的合并排序（In place），我决定用Python实现它。问题是我找不到我做错了什么......我在C ++中看到了一些代码，但即便如此我也无法修复它。
这是我的代码：
def MERGE(A,start,mid,end):
L=[0]*(mid - start + 1)
for i in range(len(L) - 1):
L[i] = A[start+i]
L[len(L) - 1] = 100000 # centinel, (fix)
R=[0]*(end - mid + 2)
for j in range(len(R) - 1):
R[j] = A[mid+j]
R[len(R) - 1] = 100000
i = 0
j = 0
k = start
for l in range(k,end):
if(L[i] < R[j]):
A[l] = L[i]
i = i + 1
else:
A[k] = R[j]
j = j + 1
def mergeSort(A,p,r):
if p < r:
mid = int((p+r)/2)
mergeSort(A,p,mid)
mergeSort(A,mid+1,r)
MERGE(A,p,mid,r)
A = [20, 30, 15, 21, 42, 45, 31, 0, 9]
mergeSort(A,0,len(A)]
当我运行代码时，我遇到了一些索引问题：
File "myrealmerge.py", line 9, in MERGE
R[j] = A[mid+j]
IndexError: list index out of range
我知道这是一个“愚蠢的问题”，并且有一些相关的帖子，但我在那里尝试了这些建议并且它对我不起作用......
任何人都可以帮助我吗？ Ť 谢谢！
答案 0 :(得分：7)
此代码可以正常工作：
def MERGE(A,start,mid,end):
L = A[start:mid]
R = A[mid:end]
i = 0
j = 0
k = start
for l in range(k,end):
if j >= len(R) or (i < len(L) and L[i] < R[j]):
A[l] = L[i]
i = i + 1
else:
A[l] = R[j]
j = j + 1
def mergeSort(A,p,r):
if r - p > 1:
mid = int((p+r)/2)
mergeSort(A,p,mid)
mergeSort(A,mid,r)
MERGE(A,p,mid,r)
A = [20, 30, 21, 15, 42, 45, 31, 0, 9]
mergeSort(A,0,len(A))
print A
我尝试将代码中的更改降至最低。
祝你好运！（添加）
您可以使用此代码检查分割过程。
def MERGE(A,start,mid,end):
# Do nothing
pass
def mergeSort(A,p,r):
if r - p > 1:
mid = int((p+r)/2)
print A[p:mid],A[mid:r]
mergeSort(A,p,mid)
mergeSort(A,mid,r)
MERGE(A,p,mid,r)
A = [20, 30, 21, 15, 42, 45, 31, 0, 9]
mergeSort(A,0,len(A))
结果如下：
[20, 30, 21, 15] [42, 45, 31, 0, 9]
[20, 30] [21, 15]
[20] [30]
[21] [15]
[42, 45] [31, 0, 9]
[42] [45]
[31] [0, 9]
[0] [9]
这就是我们想要的。但是，'mid + 1'会产生无效结果。这是测试代码：
def MERGE(A,start,mid,end):
# Do nothing
pass
def mergeSort(A,p,r):
if r - p > 1:
mid = int((p+r)/2)
print A[p:mid],A[mid+1:r] # Changed
mergeSort(A,p,mid)
mergeSort(A,mid+1,r) # Changed
MERGE(A,p,mid,r)
A = [20, 30, 21, 15, 42, 45, 31, 0, 9]
mergeSort(A,0,len(A))
结果：
[20, 30, 21, 15] [45, 31, 0, 9]
[20, 30] [15]
[20] []
[45, 31] [9]
[45] []
（添加）
以下是使用'mid + 1'的代码：
# New indexing function that includes the right index.
def get_partial_list(origin_list, left_index, right_index): # Added
return origin_list[left_index:right_index+1]
def MERGE(A,start,mid,end):
L = get_partial_list(A,start,mid)
R = get_partial_list(A,mid+1,end)
i = 0
j = 0
k = start
for l in range(k,end+1): # changed
if j >= len(R) or (i < len(L) and L[i] < R[j]):
A[l] = L[i]
i = i + 1
else:
A[l] = R[j]
j = j + 1
def mergeSort(A,p,r):
if r - p > 0: # changed
mid = int((p+r)/2)
mergeSort(A,p,mid)
mergeSort(A,mid+1,r) # changed
MERGE(A,p,mid,r)
A = [20, 30, 21, 15, 42, 45, 31, 0, 9]
mergeSort(A,0,len(A)-1) # changed
print A
我添加了新的索引功能。这是你期望的代码吗？
答案 1 :(得分：0)
将数组递归拆分为左右部分，然后根据您的要求合并，ASC或DESC，请检查以下代码：
def merge_sort(a):
if len(a) <= 1:
return a
left = [];
right = [];
mid = len(a)/2
left = a[0:mid]
right = a[mid:(len(a))]
print left
print right
#exit()
left = merge_sort(left)
right = merge_sort(right)
return merge(left, right)
def merge(left, right):
result = []
while len(left) > 0 and len(right) > 0:
lv = left[0]
rv = right[0]
if lv <= rv:
result.append(lv)
left.pop(0)
else:
result.append(rv)
right.pop(0)
while len(left) > 0:
result.append(left.pop(0))
while len(right) > 0:
result.append(right.pop(0))
return result
A = [20, 30, 21, 15, 42, 45, 31, 0, 9]
print A
print merge_sort(A)
对于理解我已经打印过中间分区数组了解输出：
[20, 30, 21, 15, 42, 45, 31, 0, 9]
[20, 30, 21, 15]
[42, 45, 31, 0, 9]
[20, 30]
[21, 15]
[20]
[30]
[21]
[15]
[42, 45]
[31, 0, 9]
[42]
[45]
[31]
[0, 9]
[0]
[9]
[0, 9, 15, 20, 21, 30, 31, 42, 45]
希望这有助于您理解逻辑。
答案 2 :(得分：0)
lancif和krishna Prasad提供的解决方案不具有空间复杂度O（1）。
这里是具有空间复杂度O（1）的Py3的就地合并排序算法。
主要功能sort_imerge使用2个辅助功能wmerge和wsort。
此解决方案基于以下讨论的C代码： other SO topic和good C implementation上 您还可以使用doctests here
找到完整的Python代码def sort_imerge(Seq, l=0, u=None):
""" Merge sorting, mutable input.
Input sequence changed in place.
Time: O(n * log n)
log n -- levels
n -- elements on each level must be merged
Space (additional): O(1)
input changed in place
Returns None
"""
u = len(Seq) if u is None else u
if u - l > 1:
m = l + (u - l) // 2
w = l + u - m
wsort(Seq, l, m, w)
while w - l > 2:
n = w
w = l + (n - l + 1) // 2
wsort(Seq, w, n, l)
wmerge(Seq, l, l + n - w, n, u, w)
n = w
while n > l: # fallback to insert sort
for m in range(n, u):
if Seq[m-1] > Seq[m]:
Seq[m-1], Seq[m] = Seq[m], Seq[m-1]
n -= 1
def wmerge(Seq, i, m, j, n, w):
"""Merge subarrays [i, m) and [j, n) into work area w.
All indexes point into Seq.
The space after w must be enough to fit both subarrays.
"""
while i < m and j < n:
if Seq[i] < Seq[j]:
Seq[i], Seq[w] = Seq[w], Seq[i]
i += 1
else:
Seq[j], Seq[w] = Seq[w], Seq[j]
j += 1
w += 1
while i < m:
Seq[i], Seq[w] = Seq[w], Seq[i]
i += 1
w += 1
while j < n:
Seq[j], Seq[w] = Seq[w], Seq[j]
j += 1
w += 1
def wsort(Seq, l, u, w):
"""Sort subarray [l, u) and put reuslt into work area w.
All indexes point into Seq.
"""
if u - l > 1:
m = l + (u - l) // 2
sort_imerge(Seq, l, m)
sort_imerge(Seq, m, u)
wmerge(Seq, l, m, m, u, w)
else:
while l < u:
Seq[l], Seq[w] = Seq[w], Seq[l]
l +=1
w +=1