LeetCode 136 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?

class Solution:
    # @param A, a list of integer
    # @return an integer
    def singleNumber(self, A):
        d={}
        for x in A:
            if not x in d:
                d[x]=1
            else:
                d[x]+=1
        for x in A:
            if d[x]==1:
                return x