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[leetcode] 347. Top K Frequent E

[leetcode] 347. Top K Frequent E

作者: 叶孤陈 | 来源:发表于2017-06-06 17:08 被阅读0次

    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
    本题直观想法是用哈希表统计元素个数,然后用堆来找出前K个数量最多的元素。具体代码如下:

    class Solution {
        typedef pair<int,int> data;
    public:
        vector<int> topKFrequent(vector<int>& nums, int k) {
            unordered_map<int,int> hash;
            priority_queue<data, vector<data>, greater<data>> heap; 
            vector<int > ret;
            
            for(int num:nums)
                hash[num]++;
                
            for(auto it:hash)
            {
                heap.push(make_pair(it.second,it.first));
                if(heap.size() > k) heap.pop();
            }
            
            for(int i = 0; i < k; ++i)
            {
                ret.push_back(heap.top().second);
                heap.pop();
            }
            return ret;
            
        }
    };
    

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