Check If Array Pairs Are Divisible by k

Given an array of integers arr of even length n and an integer k.

We want to divide the array into exactly n / 2 pairs such that the sum of each pair is divisible by k.

Return true If you can find a way to do that or false otherwise.

LeetCode Problem - 1497

class Solution {
    public boolean canArrange(int[] arr, int k) {
        // Array to keep track of the frequency of remainders when elements are divided by k
        int[] freq = new int[k];

        // Loop through the array to calculate remainders and update their frequencies
        for(int num : arr){
            // Calculate remainder ensuring it's non-negative
            int remainder = (num % k + k) % k; 
            freq[remainder]++; // Increment the frequency of the remainder
        }

        // If the frequency of remainder 0 is odd, it's not possible to pair the elements
        if(freq[0] % 2 != 0) return false;

        // Loop through the remainders from 1 to k/2 (to check both sides of the remainder spectrum)
        for(int rem = 1; rem <= k / 2; rem++){
            int counterHalf = k - rem; // Find the counterpart remainder that would sum to a multiple of k

            // If the frequency of a remainder is not equal to the frequency of its counterpart
            // it means pairs cannot be formed
            if(freq[rem] != freq[counterHalf]){
                return false;
            }
        }

        // If all checks passed, return true
        return true;
    }
}