1497. Check If Array Pairs Are Divisible by k

Problem

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**.

  Example 1:

Input: arr = [1,2,3,4,5,10,6,7,8,9], k = 5
Output: true
Explanation: Pairs are (1,9),(2,8),(3,7),(4,6) and (5,10).

Example 2:

Input: arr = [1,2,3,4,5,6], k = 7
Output: true
Explanation: Pairs are (1,6),(2,5) and(3,4).

Example 3:

Input: arr = [1,2,3,4,5,6], k = 10
Output: false
Explanation: You can try all possible pairs to see that there is no way to divide arr into 3 pairs each with sum divisible by 10.

  Constraints:

• arr.length == n

• 1 <= n <= 10^5

• n is even.

• -10^9 <= arr[i] <= 10^9

• 1 <= k <= 10^5

Solution (Java)

class Solution {
    public boolean canArrange(int[] arr, int k) {
        int[] freq = new int[k];
        for (int num : arr) {
            freq[Math.abs((num % k) + k) % k]++;
        }
        if (freq[0] % 2 != 0) {
            return false;
        }
        for (int i = 1; i <= k / 2; i++) {
            if (i == k - i && freq[i] % 2 != 0) {
                return false;
            }
            if (freq[i] != freq[k - i]) {
                return false;
            }
        }
        return true;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).