Find the Duplicate Number Problem Statement

Given an integer array 'ARR' of size 'N' containing numbers from 0 to (N - 2). Each number appears at least once, and there is one number that appears twice. Your task is to find and return this duplicate number.

Example:

Input:

t = 1
N = 5
ARR = [0, 3, 1, 2, 3]

Output:

3

Explanation:

The number '3' is the duplicate because it appears twice in the array.

Constraints:

• 1 <= t <= 10^2
• 0 <= N <= 10^3
• Time Limit: 1 sec

Note:

The duplicate number is always present in the given array.