new_contents: """
  Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements
  appear twice and others appear once.
  
  Find all the elements of [1, n] inclusive that do not appear in this array.
  
  Could you do it without extra space and in O(n) runtime? You may assume the
  returned list does not count as extra space.
  
  Example:
  
      Input:
      [4,3,2,7,8,2,3,1]
  
      Output:
      [5,6]
  """
  class Solution(object):
      def findDisappearedNumbers(self, nums):
          """
          :type nums: List[int]
          :rtype: List[int]
          """
          soln = []
          nset = set(nums)
          for i in range(1, len(nums) + 1):
              if i not in nset:
                  soln.append(i)
          return soln
  
  assert Solution().findDisappearedNumbers([4,3,2,7,8,2,3,1]) == [5,6]
  print("448 all test cases passed")
  
subject: add 448