// Single Number — EASY
// Category: array
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 linear runtime complexity and use only constant extra space.
Hint: XOR is your friend — `a ^ a = 0` and `a ^ 0 = a`, so XOR-ing all elements cancels duplicates.
Example: nums = [2, 2, 1]
Output: 1