Single Number
Description: In array nums every element appears twice except for one which appears once. Find that element.
Example 1:
Input: nums = [2,2,1]
Output: 1
Explanation: Two appears twice, one appears once
Example 2:
Input: nums = [4,1,2,1,2]
Output: 4
Explanation: Only four has no pair
Constraints:
1 <= nums.length <= 3·10⁴
Каждый элемент кроме одного встречается ровно дважды
Recommended time and space complexity
Aim for O(n) time and **O(1) space** — no extra data structures.
Hint 1
A hash table works but needs O(n) space. The constraint asks for O(1).
Hint 2
Recall XOR properties: x ^ x = 0 and x ^ 0 = x.
Hint 3
XOR is commutative, so order does not matter. Pairs cancel out.