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?

要求：O(n)时间复杂度，O(1)空间复杂度。
思路：使用异或的交换性和结合性:如果数组为 {2,1,4,5,2,4,1}
2^1^4^5^2^4^1=>(2^2)^(1^1)^(4^4)^5=>0^0^0^5=>5
参考https://discuss.leetcode.com/topic/1916/my-o-n-solution-using-xor