arrays - Why doesn't this python implementation of mergesort work? -

when input [8,7,6,5,4,3,2,1] output => [4, 3, 2, 1, 8, 7, 6, 5].

it seems thing different working solution (comparing here) instead of sorted list, have k variable incrementing, , update arr[k] in place of sorted.

why doesn't work? , how updating arr[k] work? seems losing data updating original input array.

def mergesort(arr):     if len(arr) == 1:         return      else:         mid = len(arr)/2         left = arr[0:mid]         right = arr[mid:len(arr)]          sorted = []         = 0         j = 0         mergesort(left)         mergesort(right)          while < len(left) , j < len(right):             if left[i] < right[j]:                 sorted.append(left[i])                 += 1             else:                 sorted.append(right[j])                 j += 1          while < len(left):             sorted.append(left[i])             += 1          while j < len(right):             sorted.append(right[j])             j += 1          return sorted 

you should assign left , right variable function return sorted list after sorting in base case should return list , use // integer division check code

def mergesort(arr):     if len(arr) == 1:         return arr      else:         mid = len(arr)//2         left = arr[0:mid]         right = arr[mid:len(arr)]          sorted = []         = 0         j = 0         left = mergesort(left) #left sorted         right = mergesort(right)          while < len(left) , j < len(right):             if left[i] < right[j]:                 sorted.append(left[i])                 += 1             else:                 sorted.append(right[j])                 j += 1          while < len(left):             sorted.append(left[i])             += 1          while j < len(right):             sorted.append(right[j])             j += 1          return sorted  print (mergesort([8,7,6,5,4,3,2,1,3]))