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.
用unordered_map记录每个数出现的次数，然后用优先队列挑选出出现次数最多的前k个数。

代码：

class Solution {
public:
vector<int> topKFrequent(vector<int>& nums, int k)
{
vector<int>res;
unordered_map<int,int>m;
for(int i=0;i<nums.size();i++) m[nums[i]]++;
unordered_map<int,int>::iterator it;
priority_queue<pair<int,int> >q;
for(it =m.begin();it!=m.end();it++)
{
q.push(make_pair(it->second,it->first) );
if(q.size()>(int)m.size()-k)
{
res.push_back(q.top().second);
q.pop();
}
}
return res;
}
};