归并排序求逆序対

hdu  4491



#include <iostream>
#include<stdio.h>
#include<algorithm>
#include<cmath>
using namespace std;
int a[100005];
int b[100005];
long long int ans;
void Merge (int l,int t,int r)
{
    int i=l;
    int j=t+1;
    int k=l;
     while(i <= t && j <= r)
     {

         if(a[i]>a[j])
        {
            b[k++] = a[j++];
            ans += t- i + 1;
        }
        else
        {
            b[k++] = a[i++];
        }
    }

    while(i <= t) b[k++] = a[i++];
    while(j <= r) b[k++] = a[j++];
    for(int i=l;i<=r;i++)
        a[i] = b[i];
}
void Merge_sort(int l,int r)
{
    if(r>l)
    {
        int t=(r+l)/2;
        Merge_sort(l,t);
        Merge_sort(t+1,r);
        Merge(l,t,r);
    }
}
int main()
{

int n,m;
while(~scanf("%d%d",&n,&m))
{

   for(int i=1;i<=n;i++)
   {
       scanf("%d",&a[i]);

   }
    ans=0;
   Merge_sort(1,n);
   cout<<max(ans-m,(__int64)0)<<endl;
}
    return 0;
}