LC 347 K most frequent - C++ - bucket sort - code blog: http://storypku.com/2016/05/leetcode-question-347-top-k-frequent-elements/

LC347KMostFrequentBucketSort2.cpp

public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        std::unordered_map<int, int> mapping;
        for(auto v: nums) {
            mapping[v] ++;
        }
        std::vector<vector<int> > buckets(nums.size() + 1);
        for(auto ipair: mapping) {
            buckets[ipair.second].push_back(ipair.first);
        }
        
        std::vector<int> result;
        
        for(int i = buckets.size() - 1; i >= 0 && result.size() < k; --i) {
            for(auto elem: buckets[i]) {
                result.push_back(elem);
                if (result.size() == k)
                    break;
            }        
        }
        return result;
    }
};