leetcode 347. Top K Frequent Elements

347. Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

For example,
Given [1,1,1,2,2,3] and k = 2, return [1,2].

Note:

• You may assume k is always valid, 1 ≤ k ≤ number of unique elements.

• Your algorithm's time complexity must be better than O(n log n), where n is the array's size.

解题思路

O(N + N*longN) 的解法

先用无序哈希表记录数字出现次数，然后插入到最大堆中，最后从堆中取出前 K 个元素并返回。

struct FrequencyCmp {
	bool operator() (const pair<int, unsigned int> &lhs, pair<int, unsigned int> &rhs) {
		return  lhs.second < rhs.second ? true : false;
	}
};

class Solution {
public:
	vector<int> topKFrequent(vector<int>& nums, int k) {
		unordered_map<int, unsigned int> numberFrequency;

		for (auto e : nums) {
			numberFrequency[e]++;
		}

		priority_queue<pair<int, unsigned int>, vector<pair<int, unsigned int>>, FrequencyCmp> heap;

		for (auto e : numberFrequency) {
			heap.push(e);
		}

		vector<int> v;

		for (int i = 0; i < k; i++) {
			v.push_back(heap.top().first);
			heap.pop();
		}

		return v;
	}
};