https://leetcode.com/problems/single-number/description/
Given a non-empty array of integers nums, every element appears twice except for one. Find that single one.
You must implement a solution with a linear runtime complexity and use only constant extra space.
Input: nums = [2,2,1]
Output: 1
Input: nums = [4,1,2,1,2]
Output: 4
Input: nums = [1]
Output: 1
1 <= nums.length <= 3 * 104
-3 * 104 <= nums[i] <= 3 * 104
Each element in the array appears twice except for one element which appears only once.