old_contents: # Time:  O(n)
  # Space: O(1)
  
  class Solution(object):
      def missingNumber(self, nums):
          """
          :type nums: List[int]
          :rtype: int
          """
          return reduce(operator.xor, nums, \
                        reduce(operator.xor, xrange(len(nums) + 1)))
  
new_contents: # Time:  O(n)
  # Space: O(1)
  #
  # Given an array containing n distinct numbers taken from
  # 0, 1, 2, ..., n, find the one that is missing from the array.
  #
  # For example,
  # Given nums = [0, 1, 3] return 2.
  #
  # Note:
  # Your algorithm should run in linear runtime complexity. 
  # Could you implement it using only constant extra space complexity?
  #
  
  class Solution(object):
      def missingNumber(self, nums):
          """
          :type nums: List[int]
          :rtype: int
          """
          return reduce(operator.xor, nums, \
                        reduce(operator.xor, xrange(len(nums) + 1)))
  
subject: Update missing-number.py