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?

不用bit操作只能做到liner runtime complexity. 没法弄成no extra memory.

class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        dict = {}
        for n in nums:
            if not dict.pop(n,None):
                dict[n] = 1
        for k in dict:
            return k