347. Top K Frequent Elements

思路：
先建立hash表统计出现的次数. 然后使用桶排序, 按照次数丢入桶中. 时间复杂度O(n)

code:

class Solution {public:    vector<int> topKFrequent(vector<int>& nums, int k) {        map<int, int> hash;        for (int i = 0; i < nums.size(); i++)            hash[nums[i]]++;        vector<list<int>> buckets(nums.size() + 1);        for (map<int, int>::iterator it = hash.begin(); it != hash.end(); ++it)  // replace to auto            buckets[it->second].push_back(it->first);        vector<int> result;        for (int i = nums.size(); i >= 0 && result.size() < k; i--)            //for(list<int>::iterator it = buckets[i].begin(); it != buckets[i].end() ; it++) {            for (auto it : buckets[i]) {  // STL auto since C++11                result.push_back(it);                if (result.size() == k)                    break;            }        return result;    }};