Given a non-empty list of integers where every element appears exactly twice except for one element which appears exactly once, find and return that unique element. Your solution should not use extra memory proportional to the input size.
Examples:
Input: nums = [2, 2, 1]
Output: 1
Explanation: 1 appears once, 2 appears twice
Input: nums = [4, 1, 2, 1, 2]
Output: 4
Explanation: 4 appears once, 1 and 2 each appear twice
Input: nums = [1]
Output: 1
Explanation: Only one element, it is the single number
Input: nums = [3, 3, 7, 7, 5]
Output: 5
Explanation: 5 appears once, 3 and 7 each appear twice
Input: nums = [0, 1, 0]
Output: 1
Explanation: 1 appears once, 0 appears twice
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