TopK

    面试到了一个topk，这个原理很简单，但是以前很少写过。面试时写的有点小慢，没有达到行云流水的地步。于是回来再写一遍练练。其中，堆排序部分采用简明排序代码。用完整的TopK代码：

#include <iostream>
#include <algorithm>

using namespace std;

template<typename T>
void unguarded_heapify(T *data, size_t size, size_t top)
{
    while (true)
    {
        size_t min = top;

        if (top * 2 < size && data[top * 2] < data[min])
        {
            min = top * 2;
        }

        if (top * 2 + 1 < size && data[top * 2 + 1] < data[min])
        {
            min = top * 2 + 1;
        }

        if (top == min) return;

        swap(data[top], data[min]);
        top = min;
    }
}

template<typename T>
void make_min_heap(T *begin, T *end)
{
    if (begin == NULL || end == NULL)
    {
        return;
    }

    if (begin == end || begin + 1 == end)
    {
        return;
    }

    size_t len = end - begin;

    for (size_t top = len / 2; top >= 1; --top)
    {
        // Special offset.
        unguarded_heapify(begin - 1, len + 1, top);
    }
}

void topk(const int *begin, const int *end, int *buffer, size_t *k)
{
    if (begin == NULL || end == NULL || buffer == NULL || k == NULL)
    {
        return;
    }

    if (begin == end || *k == 0)
    {
        return;
    }

    memset(buffer, 0, *k * sizeof(int));

    const int *p = begin;
    int *dest = buffer;

    while (p != begin + *k && p != end)
    {
        *dest++ = *p++;
    }

    if (p == end)
    {
        *k = end - begin;
    }
    else
    {
        make_min_heap(buffer, dest);

        while (p != end)
        {
            if (*p > *buffer)
            {
                *buffer = *p;
                unguarded_heapify(buffer - 1, *k + 1, 1);
            }

            ++p;
        }
    }
}

int main(int argc, char **argv)
{
    int data[] = {4, 5, 1, 3, 5, 6, 7, 2};
    int *result = new int[10];

    size_t k = 3;
    topk(data, data + sizeof(data) / sizeof(data[0]), result, &k);
    copy(result, result + k, ostream_iterator<int>(cout, " "));
    cout << endl;

    k = 10;
    topk(data, data + sizeof(data) / sizeof(data[0]), result, &k);
    copy(result, result + k, ostream_iterator<int>(cout, " "));
    cout << endl;

    k = 1;
    topk(data, data + sizeof(data) / sizeof(data[0]), result, &k);
    copy(result, result + k, ostream_iterator<int>(cout, " "));
    cout << endl;

    k = 8;
    topk(data, data + sizeof(data) / sizeof(data[0]), result, &k);
    copy(result, result + k, ostream_iterator<int>(cout, " "));
    cout << endl;

    k = 0;
    topk(data, data + sizeof(data) / sizeof(data[0]), result, &k);
    copy(result, result + k, ostream_iterator<int>(cout, " "));
    cout << endl;

    delete[] result;
    return 0;
}