# 347. Top K Frequent Elements

## 1.問題

* 給予一個整數序列, 回傳前k個出現次數最多的成員

![](/files/-LItoDORB99CcoUq93Ek)

## 2.想法

* Bucket sort
  * 先計算每個數字出現的次數並推到無序容器, ex: unordered\_map中
  * 以次數為索引, 將(索引, 數字)推入有序器如vector中, 考慮相同出現次數的情況, 這邊可以用vector\<vector\<int>>
  * 將排在後面的成員推入回傳的序列中

## 3.程式碼

```
class Solution {
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
        if (nums.size() == 1)
        {
            return nums;
        }

        //Count
        unordered_map<int, int> cnts;
        for (auto num: nums) {
            cnts[num]++;
        }
        
        //Push into buckets
        vector<vector<int>> bucket(nums.size() + 1);
        for (auto cnt : cnts) {
            bucket[cnt.second].push_back(cnt.first);
        }
        
        //Push the higher index into res
        vector<int> res;
        int index = 0;
        for (int i = bucket.size() - 1 ; i >= 0; i--) {
            if (bucket[i].size() != 0 && index < k)
            {
                for (auto v: bucket[i]) {
                    index++;
                    res.push_back(v);
                }
            }
        }
        return res;
    }
};
```

## 4.Performance

![](/files/-LItoY8qslzZT60NFVdD)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jenhsuan.gitbook.io/algorithm/leetcode/347.-top-k-frequent-elements.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.