Unique Element in Array

Given an arbitrary array arr consisting of N non-negative integers where every element appears thrice except for one. Your task is to find the element in the array that appears only once.

Input:

First line: Single integer T indicating the number of test cases.
Next 2 * T lines: For each test case,
- A single integer N denoting the number of elements in the array.
- N space-separated integers representing the elements of the array.

Output:

For each test case, output a single integer — the element that appears only once.

Example:

Input:

T = 2
arr1 = [2, 2, 3, 2]
arr2 = [0, 1, 0, 1, 0, 1, 99]

Output:

3
99

Explanation:

In the first test case, every element appears thrice except for 3, which appears once. In the second test case, the element 99 appears only once.

Constraints:

• 1 ≤ T ≤ 100
• 4 ≤ N ≤ 104
• 0 ≤ arr[i] < 109

Note:

No need to print anything; just implement the function to return the answer.

Follow Up:

Try to achieve O(N) time complexity and O(1) space complexity.