[LeetCode] 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.

问题描述：

给定一非空整数数组，返回频率最高的k个元素。算法的时间复杂度必须要优于O(nlogn)

class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k) {
unordered_map<int, int> cnt;
for (auto num : nums)
cnt[num]++; // 使用unordered_map进行计数

auto cmp = [](pair<int, int> &p1, pair<int, int> &p2) { return p1.second < p2.second; };
priority_queue<pair<int, int>, vector<pair<int, int>>, decltype(cmp)> pq(cmp);
for (auto &p : cnt)
pq.push(p); // 优先队列，进行排序。

vector<int> output;
while (k > 0) {
auto p = pq.top();
pq.pop();
k--;
output.push_back(p.first);
}

return output;
}
};