leetcode 136-Single Number

难度：easy

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?

思路：1. 这种找出list里面唯一一个重复次数与其他都不同的element的题特别适合用dictionary来做。

           2. 将list里面的每个element变成dictionary里的key，element在list里面出现的次数则为value，最后找到value=1时对应的key即可。

           3. dictionary操作：