Number of Good Pairs

Problem statement

Given an array of integers nums, return the number of good pairs.

A pair (i, j) is called good if nums[i] == nums[j] and i < j.

Example 1:

Input: nums = [1,2,3,1,1,3]Output: 4Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]Output: 6Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]Output: 0

Constraints:

• 1 <= nums.length <= 100
• 1 <= nums[i] <= 100

My solution

/**
 * @param {number[]} nums
 * @return {number}
 */
var numIdenticalPairs = function(nums) {
    const track = {};
    for (let i = 0; i < nums.length; i++) {
        if (!track[nums[i]]) {
            track[nums[i]] = [i]
        } else {
            track[nums[i]].push(i)
        }
    }
    
    const trackFac = {};
    function fact(n) {
        if (n === 0 || n === 1) {
            return 1;
        }

        if (trackFac[n]) {
            return trackFac[n];
        } else {
            trackFac[n] = fact(n - 1) * n
            return trackFac[n]
        }
    }
    
    return Object.keys(track).reduce((acc, curr) => {
        if (track[curr] && track[curr].length > 1) {
            // console.log(track[curr])
            const len = track[curr].length;
            acc += (fact(len)/(fact(2) * fact(len - 2)))
        }
        
        return acc;
    }, 0)
};