message: Update concatenation-of-array.py
diff: # Space: O(1)
  class Solution(object):
  + def getConcatenation(self, nums):
  + """
  + :type nums: List[int]
  + :rtype: List[int]
  + """
  + nums.extend(nums)
  + return nums
  +
  +
  +# Time: O(n)
  +# Space: O(1)
  +class Solution2(object):
  def getConcatenation(self, nums):
  """
  :type nums: List[int]
  :rtype: List[int]
  """
  return nums+nums
  +
  +
  +# Time: O(n)
  +# Space: O(1)
  +class Solution3(object):
  + def getConcatenation(self, nums):
  + """
  + :type nums: List[int]
  + :rtype: List[int]
  + """
  + return nums*2
  