new_contents: """
  Question:
      Single Number
  
      Given an 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?
  
  Performance:
      1. Total Accepted: 92009 Total Submissions: 199999 Difficulty: Medium
      2.  Your runtime beats 55.22% of python submissions.
  """
  
  
  class Solution(object):
      def singleNumber(self, nums):
          """
          :type nums: List[int]
          :rtype: int
  
          Google it, and suddenly understand that `xor` let two same number convert all bits to zero, so the single one lefted.
          """
          result = 0
  
          for num in nums:
              result ^= num
  
          return result
  
  
  assert Solution().singleNumber([1, 0, 1]) == 0
  assert Solution().singleNumber([1, 3, 1]) == 3
  
subject: Add 136-single-number.py