Find the non-duplicate number

Hi, here's your problem today. This problem was recently asked by Facebook:

Given a list of numbers, where every number shows up twice except for one number, find that one number.

Example:
Input: [4, 3, 2, 4, 1, 3, 2]
Output: 1
Here's the function signature:

def singleNumber(nums):

  # Fill this in.



print singleNumber([4, 3, 2, 4, 1, 3, 2])

# 1

Challenge: Find a way to do this using O(1) memory.