message: Update missing-number-in-arithmetic-progression.py
diff: @@ -19,3 +19,14 @@ class Solution(object):
  else:
  left = mid+1
  return arr[0] + d*left
  +
  +
  +# Time: O(n)
  +# Space: O(1)
  +class Solution2(object):
  + def missingNumber(self, arr):
  + """
  + :type arr: List[int]
  + :rtype: int
  + """
  + return (min(arr)+max(arr))*(len(arr)+1)//2 - sum(arr)
  