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fix: correct recursive merge sort implementation #1

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fix: correct recursive merge sort implementation #1

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fix: correct recursive merge sort implementation
bterwijn Jun 11, 2026
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bterwijn Jun 11, 2026
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94 changes: 49 additions & 45 deletions sorting-algorithms/merge_sort_recursive.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,61 +2,65 @@

# lst[low1]...lst[up1] and lst[low2]...lst[up2] merged to temp[low1]...temp[up2]
def merge(lst, temp, low1, up1, low2, up2):
i = low1
j = low2
k = low1

while i<=up1 and j<=up2:
if lst[i] < lst[j]:
temp[k] = lst[i]
i += 1
else:
temp[k] = lst[j]
j += 1
k += 1

while i <= up1:
temp[k] = lst[i]
k += 1
i += 1

while(j <= up2):
temp[k] = lst[j]
k += 1
j += 1
i = low1
j = low2
k = low1

while i <= up1 and j <= up2:
if lst[i] <= lst[j]:
temp[k] = lst[i]
i += 1
else:
temp[k] = lst[j]
j += 1
k += 1

while i <= up1:
temp[k] = lst[i]
k += 1
i += 1

while j <= up2:
temp[k] = lst[j]
k += 1
j += 1


def copy(lst, temp, low, up):
i = low
while i<=up:
lst[i] = temp[i]
i += 1
i = low
while i <= up:
lst[i] = temp[i]
i += 1

def merge_sort1(lst, low, up):
temp = [None]*(len(lst))

if low == up: # if only one element
return

mid = (low+up)//2
merge_sort1(lst,low,mid) # Sort lst[low]....lst[mid]
merge_sort1(lst,mid+1,up) # Sort lst[mid+1]....lst[up]
def merge_sort1(lst, temp, low, up):
if low >= up: # if only one element
return

# Merge lst[low]...lst[mid] and lst[mid+1]...lst[up] to temp[low]...temp[up]
merge(lst,temp,low,mid,mid+1,up)
mid = (low + up) // 2

merge_sort1(lst, temp, low, mid) # Sort lst[low]....lst[mid]
merge_sort1(lst, temp, mid + 1, up) # Sort lst[mid+1]....lst[up]

# Merge lst[low]...lst[mid] and lst[mid+1]...lst[up] to temp[low]...temp[up]
merge(lst, temp, low, mid, mid + 1, up)

# Copy temp[low]...temp[up] to lst[low]...lst[up]
copy(lst, temp, low, up)

# Copy temp[low]...temp[up] to lst[low]...lst[up]
copy(lst,temp,low,up)

def merge_sort(lst, n):
merge_sort1(lst,0,n-1)
temp = [None] * n
merge_sort1(lst, temp, 0, n - 1)


if __name__ == '__main__':
data_list = [8, 5, 89, 30, 42, 92, 64, 4, 21, 56, 3]
data_list = [8, 5, 89, 30, 42, 92, 64, 4, 21, 56, 3]

print("Unsorted list is :")
print(data_list)
print("Unsorted list is :")
print(data_list)

merge_sort(data_list,len(data_list))
merge_sort(data_list, len(data_list))

print("Sorted list is :")
print(data_list)
print("Sorted list is :")
print(data_list)