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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?使用异或运算，XOR！！！
大杀器！！！本来我想使用unordered_map的，但是需要额外的空间。
class Solution { //using xorpublic: int singleNumber(vector & nums) { int res=nums[0]; for(int i=1;i 转载地址：http://eexvi.baihongyu.com/