new_contents: 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?
  
  class Solution:
      # @param {integer[]} nums
      # @return {integer}
      def singleNumber(self, nums):
          pre = nums[0]
          
          for num in nums[1:]:
              pre = num ^ pre
          
          return pre 
  
subject: Create Single_NumberI.py