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?
可以巧妙地运用XOR运算符，因为两个相同的数字XOR之后会被消除影响，所以剩下的就是唯一存在的数字。
原文：http://www.cnblogs.com/nano94/p/4363061.html