TopKFrequentElements

// Author: Sayed Mahmudul Alam
// Source: LeetCode-347: https://leetcode.com/problems/top-k-frequent-elements/description/
// Solution Ref: https://www.youtube.com/watch?v=YPTqKIgVk-k&ab_channel=NeetCode
// Example Input: nums = [1,1,1,2,2,3], k = 2
// Output: [1,2]

class Solution {
    func topKFrequent(_ nums: [Int], _ k: Int) -> [Int] {

        // Dictionary to hold count of each element
        var count = [Int: Int]()

        // Frequency table to hold the frequency of each number
        var freq = [[Int]](repeating: [], count: nums.count + 1)

        // Array to hold the result
        var result = [Int]()

        // Populating the `count` dictionary with actual counts
        nums.forEach { n in
            count[n] = 1 + (count[n] ?? 0)
        }

        // Populate the `freq` table with elements for each number
        for (_,value) in count.enumerated() {
            freq[value.1].append(value.0)
        }

        // Loop through `freq` in reverse and add the element of inner array to `result`
        // till `result` is equal `k`
        let sequence = stride(from: freq.count - 1, to: 0, by: -1)
        for i in sequence {
            for j in freq[i] {
                result.append(j)
                if result.count == k {
                    return result
                }
            } 
        }

        return []
    }
}