package LeetCode_347 import java.util.* import kotlin.collections.ArrayList import kotlin.collections.HashMap /** * 347. Top K Frequent Elements * https://leetcode.com/problems/top-k-frequent-elements/description/ * * Given a non-empty array of integers, return the k most frequent elements. * Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = [1], k = 1 Output: [1] Time complexity try on O(n) * */ class Solution { fun topKFrequent(nums: IntArray, k: Int): List<Int> { val result = ArrayList<Int>() //first: num //second: the frequency of num val maxHeap = PriorityQueue<Pair<Int,Int>>(kotlin.Comparator { a, b -> b.second - a.second }) val frequencyMap = HashMap<Int, Int>() for (num in nums) { frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1) } frequencyMap.forEach { key, value -> //value is frequency val pair = Pair(key,value) maxHeap.add(pair) } for (i in 0 until k){ result.add(maxHeap.remove().first) } return result } }