求逆序数

POJ2299 Ultra-sort可以转成求逆序数。

如果规模比较小,求逆序数可以用冒泡的方法做,简单。还可以用归并排序做,先分别求左右两部分的逆序数,两部分都排好序后再求两者之间的逆序数。因此求逆序数是归并排序的副产品。与归并排序的主要差别也就在一句ret += (a + m - posa);

 

没有ac,惭愧。

 

#include <cstdio>
#include <vector>

using namespace std;

__int64 mergearr(int* a, int m, int* b, int n)
{
    __int64 ret = 0;
    int* assit = new int[m + n];
    int* posa = a, *posb = b, *cur = assit;
    while (posa < a + m && posb < b + n)
    {
        if (*posa <= *posb)
        {
            *cur = *posa;
            ++cur;
            ++posa;
        }
        else
        {
            *cur = *posb;
            ++cur;
            ++posb;
            ret += (a + m - posa);
        }
    }

    if (posa == a + m)
    {
        for (; posb < b + n; ++posb,++cur)
        {
            *cur = *posb;
        }
    }
    else
    {
        for (; posa < a + m; ++posa, ++cur)
        {
            *cur = *posa;
        }
    }

    memcpy(a, assit, sizeof(int) * (m + n));
    delete assit;
    return ret;
}

__int64 mergesort(int* base, int n)
{
    if (n == 1)
    {
        return 0;
    }

    int half = (n >> 1);
    __int64 ret = mergesort(base, half);
    ret += mergesort(base + half, n - half);
   
    ret += mergearr(base, half, base + half, n - half);
}

int main()
{
    int i,n;
    vector<int> vec;
    vec.resize(500001);

    for (; ;)
    {
        scanf("%d", &n);
        if (!n)
        {
            break;
        }

        for (i = 0; i < n; ++i)
        {
            scanf("%d", &vec[i]);
        }

        __int64 res = mergesort(&vec[0], n);
        printf("%I64d/n", res);
    }
    return 0;
}