Count_Equal_and_Divisible_Pairs_in_an_Array_By_AI.js

/*
Given a 0-indexed integer array nums of length n and an integer k, return the number of pairs (i, j) where 0 <= i < j < n, such that nums[i] == nums[j] and (i * j) is divisible by k.
 

Example 1:

Input: nums = [3,1,2,2,2,1,3], k = 2
Output: 4
Explanation:
There are 4 pairs that meet all the requirements:
- nums[0] == nums[6], and 0 * 6 == 0, which is divisible by 2.
- nums[2] == nums[3], and 2 * 3 == 6, which is divisible by 2.
- nums[2] == nums[4], and 2 * 4 == 8, which is divisible by 2.
- nums[3] == nums[4], and 3 * 4 == 12, which is divisible by 2.
Example 2:

Input: nums = [1,2,3,4], k = 1
Output: 0
Explanation: Since no value in nums is repeated, there are no pairs (i,j) that meet all the requirements.
 

Constraints:

1 <= nums.length <= 100
1 <= nums[i], k <= 100
*/
function countPairs(nums, k) {
    const groups = {};
    // Group indices by their corresponding number in nums
    for (let i = 0; i < nums.length; i++) {
        const num = nums[i];
        if (!groups[num]) {
            groups[num] = [];
        }
        groups[num].push(i);
    }

    let count = 0;
    // Check each group for valid pairs
    for (const key in groups) {
        const indices = groups[key];
        if (indices.length < 2) continue;
        // Generate all possible pairs (i, j) where i < j
        for (let i = 0; i < indices.length; i++) {
            for (let j = i + 1; j < indices.length; j++) {
                const a = indices[i];
                const b = indices[j];
                if ((a * b) % k === 0) {
                    count++;
                }
            }
        }
    }
    return count;
}