hdoj1394(归并排序)

归并排序求逆元;

#include <cstdio>
#define MAXN 5000+10
#define LL long long
#include <iostream>
using namespace std;
int a[MAXN], tmp[MAXN];
int b[MAXN];
LL ans;
void Merge(int l, int m, int r)
{
    int i=l;
    int j=m+1;
    int k=l;
    while(i<=m && j<=r)
    {
        if(a[i]> a[j])
        {
            tmp[k++]=a[j++];
            ans+=m-i+1;
        }
        else
        {
            tmp[k++]=a[i++];
        }
    }
    while(i<=m) tmp[k++]=a[i++];
    while(j<=r) tmp[k++]=a[j++];
    for(int i=l; i<=r; i++)
    {
        a[i]=tmp[i];
    }
}
void Merge_sort(int l, int r)
{
    if(l <r)
    {
        int m=(l+r)>> 1;
        Merge_sort(l, m);
        Merge_sort(m+1, r);
        Merge(l, m, r);
    }
}
int main()
{
    int n; LL res;
    while(scanf("%d", &n) != EOF)
    {
        for(int i=0; i<n; i++)
        {
            scanf("%d", &a[i]);
            b[i]=a[i];
        }
        ans =0;
        Merge_sort(0, n-1);
        res=ans;
        for(int i=0; i<n; i++)
        {
            res += n-1-b[i]-b[i];
            ans= min(ans, res);
        }
        printf("%lld\n", ans);
    }
    return 0;
}