How should I practice Count Pairs Divisible by K?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Apple onsite interview problems in FastPrep.

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Count Pairs Divisible by K

Apple● MediumFULLTIMEONSITE INTERVIEW

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Problem statement

Given an integer array nums and positive integer k, count index pairs (i,j) with i < j for which nums[i] + nums[j] is divisible by k.

Different index pairs count separately, including pairs with equal values. An element cannot pair with itself. Return the count as a signed 64-bit integer.

Function

countDivisiblePairs(nums: int[], k: int) → long

Examples

Example 1

nums = [4,3,5,1,4,5]k = 5return = 3

The two separate occurrences of four each pair with one, and the two fives form one pair.

Example 2

nums = [-1,1,2,-2]k = 3return = 4

The valid sums are zero, negative three, positive three, and zero.

Constraints

0 <= nums.length <= 10^5.
-10^9 <= nums[i] <= 10^9.
1 <= k <= 10^9.