code: def missingNumber(self, arr: List[int]) -> int:
      diff = (arr[-1] - arr[0]) // len(arr)
  
      if diff == 0:
          return arr[0]
  
      for i in range(1, len(arr)):
          if arr[i - 1] + diff != arr[i]:
              return arr[i - 1] + diff
  
label_desc: Use image node_id 1 for calling the Solution obj's underlying member method code with example usage: obj.missingNumber(arr) and returns: arr, unknown