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?

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class Solution {
public:
int singleNumber(vector<int>& nums) {
int answer=nums[0];
for(unsigned long i=1;i!=nums.size();++i)
answer=answer^nums[i];
return answer;
}
};