LeetCode : Top K Frequent Elements

Solution - O(n)

vector<int> topKFrequent(vector<int>& nums, int k)
{
    // 统计每个数字的频率
    unordered_map<int, int> rec;
    for(int i = 0; i < nums.size(); i++)
        rec[nums[i]]++;

    // 按频率存放数字
    vector<vector<int>> bucket(nums.size());
    for (auto it : rec)
        bucket[it.second - 1].push_back(it.first);

    vector<int> res;
    for (int i = bucket.size() - 1; i >= 0 && k > 0; i--)
    {
        for (int temp : bucket[i])
            res.push_back(temp);
        k -= bucket[i].size();
    }

    return res;
}