I am tryint to write a mergesort in python , but the output was different when I input different list - mergesort

Question

When my input is[6,5,4,3,2,1],the output is [1,2,3,4,5,6],
but when my input is[1,2,3,4,5,6],the output will become [1,1,1,1,1,1].
if there is a number that bigger than the number before it,the output will become wrong
def merge (arr,low,mid,high):
L=arr[low:mid+1]
R=arr[mid+1:high+1]
i=0
j=0
k=low
while i<len(L) and j<len(R):
if L[i]<=R[j]:
arr[k]=L[i]
i+=1
k+=1
else:
arr[k]=R[j]
j+=1
k+=1
while i<len(L):
arr[k]=L[i]
i+=1
k+=1
while j<len(R):
arr[k]=L[j]
j+=1
k+=1
def sort (arr,low,high):
if low<high:
mid=low+(high-low)//2
print(mid)
sort(arr,low,mid)
sort(arr,mid+1,high)
merge(arr,low,mid,high)
arr=[1,2,3,4,5]
print(arr)
sort(arr,0,len(arr)-1)
print(arr)

Answer 1

There is a problem here:
while j<len(R):
arr[k]=R[j] # this was arr[k]=L[j]
Typically merge sort has high set to ending index = 1 + last index. This means that sort will need to check for (high - low) > 1, but it eliminate the +1's.
def merge (arr,low,mid,high):
L=arr[low:mid]
R=arr[mid:high]
i=0
j=0
k=low
while i<len(L) and j<len(R):
if L[i]<=R[j]:
arr[k]=L[i]
i+=1
k+=1
else:
arr[k]=R[j]
j+=1
k+=1
while i<len(L):
arr[k]=L[i]
i+=1
k+=1
while j<len(R):
arr[k]=R[j]
j+=1
k+=1
def sort (arr,low,high):
if (high - low) > 1: # if < 2 elements, nothing to do
mid=low+(high-low)//2
sort(arr,low,mid)
sort(arr,mid,high)
merge(arr,low,mid,high)
arr=[3,2,1,5,4]
print(arr)
sort(arr,0,len(arr)) # len(arr) = ending index of arr
print(arr)