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347. Top K Frequent Elements

Medium

Given an integer array nums and an integer k, return the k most frequent elements. You may return the answer in any order.

Example 1:

Input: nums = [1,1,1,2,2,3], k = 2

Output: [1,2]

Example 2:

Input: nums = [1], k = 1

Output: [1]

Constraints:

• 1 <= nums.length <= 105
• k is in the range [1, the number of unique elements in the array].
• It is guaranteed that the answer is unique.

Follow up: Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size.

Solution

using System;
using System.Collections.Generic;
using System.Linq;

public class Solution {
    public int[] TopKFrequent(int[] nums, int k) {
        if (k == nums.Length) {
            return nums;
        } 
        //1. build dictionary
        Dictionary<int, int> dict = new Dictionary<int, int>();
        for(int i=0; i < nums.Length; i++) {
            if (!dict.ContainsKey(nums[i])) {
                dict.Add(nums[i],0);
            }  
            dict[nums[i]] +=1;
        }
        //2. build priority queue based on highest to lowest frequency
        PriorityQueue<int, int> pq = new PriorityQueue<int, int>(Comparer<int>.Create((x, y) => y.CompareTo(x)));
        foreach (var key in dict.Keys) {
            pq.Enqueue(key, dict[key]);
        }
        // 3. return top k elements from Priority Queue
        var result = new int[k];
        for (var i = 0; i < k; i++) {
            result[i] = pq.Dequeue();
        }
        return result;
    }
}

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