Find Unique Element in Array

You have been provided an integer array/list ARR of size N. Here, N is equivalent to 2M + 1 where each test case has M numbers appearing twice and one number appearing exactly once.

Your task is to identify and return this unique number that appears only once.

Example:

Input:

t = 2
N = 5
ARR = [1, 2, 3, 2, 1] 
N = 7
ARR = [4, 5, 6, 7, 7, 6, 5]

Output:

3
4

Explanation:

In the first test case, 3 is the only number that is not repeated. In the second test case, 4 is the number which appears only once.

Constraints:

• 1 ≤ t ≤ 102
• 0 ≤ N ≤ 103
• Time Limit: 1 sec

Note:

The unique element is guaranteed to be present in the array/list according to the given condition.