前K个高频元素（分治算法）

前K个高频元素（分治算法） 前K个高频元素（分治算法）

给定一个无序数列，统计数列中出现频率最高的前K个元素。

#include 
#include 
#include 
#include 

std::vector> countFrequent(std::vector& nums)
{
    std::vector> vbucket;
    std::unordered_map bucket;
    for (auto& elem : nums)
    {
        bucket[elem]++;
    }
    for (auto& elem : bucket)
    {
        vbucket.push_back(elem);
    }
    return vbucket;
}

int partition(std::vector>& vbucket, int start, int end)
{
    int pstart = start, pend = end;
    int base = vbucket[start].second;
    while(pstart < pend)
    {
        while (vbucket[pend].second <= base && pstart < pend)
        {
            pend--;
        }
        if (vbucket[pend].second > base && pstart < pend)
        {
            std::swap(vbucket[pstart++], vbucket[pend]);
        }
        while (vbucket[pstart].second >= base && pstart < pend)
        {
            pstart++;
        }
        if (vbucket[pstart].second < base && pstart < pend)
        {
            std::swap(vbucket[pstart], vbucket[pend--]);
        }
    }
    return start;
}

void quickSort(std::vector>& vbucket, int start, int end)
{
    if (start < end)
    {
        int pos = partition(vbucket, start, end);
        quickSort(vbucket, start, pos - 1);
        quickSort(vbucket, pos + 1, end);
    }
}

std::vector topKFrequent(std::vector& nums, int k)
{
    std::vector topKFreElems;
    std::vector> vbucket = countFrequent(nums);
    quickSort(vbucket, 0, vbucket.size() - 1);
    for (int i = 0; i < vbucket.size() && k; i++, k--)
    {
        topKFreElems.push_back(vbucket[i].first);
    }
    return topKFreElems;
}

void printVector(std::vector& nums)
{
    for (auto& elem : nums)
    {
        printf("%d ", elem);
    }
    printf("n");
}

int main()
{
    std::vector nums{5, 3, 1, 1, 1, 3, 73, 1};
    printVector(nums);
    std::vector topKFreElems = topKFrequent(nums, 1);
    printVector(topKFreElems);
    return 0;
}