"""
Given an array of integers, arr, where all numbers occur twice except one number
which occurs once, find the number. Your solution should ideally be O(n) time
and use constant extra space.

Example:
>>> Solution().findSingle([7, 3, 5, 5, 4, 3, 4, 8, 8])
7
"""

class Solution(object):
  def findSingle(self, nums):
    s = set()
    for i in nums:
        if i in s:
            s.remove(i)
        else:
            s.add(i)
    return s.pop()

if __name__ == "__main__":
    import doctest
    doctest.testmod()