First Missing Positive Problem Statement

You are provided with an integer array ARR of length 'N'. Your objective is to determine the first missing positive integer using linear time and constant space. This means finding the smallest positive integer that does not appear in the array. Note that the array may include negative numbers as well.

Example:

Input:

[3, 4, -1, 1]

Output:

2

Explanation:

The number 2 is the smallest positive integer that is absent from the input array.

Input:

The input begins with an integer T denoting the number of test cases.

For each test case:

- The first line contains a positive integer N indicating the size of the array.

- The second line contains N space-separated integers comprising the array 'ARR'.

Output:

For each test case, output a single integer that represents the first missing positive integer for that test case. The result for each test case should be presented on a new line.

Constraints:

• 1 <= T <= 10
• 1 <= N <= 10^5
• -10^5 <= ARR[i] <= 10^5
• Time Limit: 1 sec

Note:

You do not need to manually print the output; the function will handle it. Simply implement the logic to solve the problem.