new_contents: """
  Given a non-empty array of integers, every element appears twice except for one. Find that single one.
  
  Note:
  
  Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?
  """
  
  class Solution:
      def singleNumber(self, nums: List[int]) -> int:
          for i in range(1, len(nums)):
              nums[0] = nums[0] ^ nums[i]
          return nums[0]
subject: Add day1 of ht eleet code challenge