Inversion HDU - 4911

AC代碼

#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
const int N=1e5+10;
typedef long long ll;
ll a[N],t[N],ans;
void merge_sort(int left,int right)
{
    if(left>=right) return;
    
    int mid=(left+right)/2;
    
    int i=left,j=mid+1,k=0;
    merge_sort(left,mid);
    merge_sort(mid+1,right);
    while(i<=mid && j<=right)
    {
        if(a[i]<=a[j]) t[k++]=a[i++];
        else
        {
            t[k++]=a[j++];
            ans+=mid-i+1;
        }
    }
    while(i<=mid) t[k++]=a[i++];
    while(j<=right) 
    {
        t[k++]=a[j++];
        ans+=mid-i+1;
    }
    for(i=left,j=0;i<=right;i++,j++)
        a[i]=t[j];
}
int main()
{
    int n,k;
    while(scanf("%d %d",&n,&k)!=EOF)
    {
        for(int i=0;i<n;i++)    scanf("%lld",&a[i]);
        ans=0;
        merge_sort(0,n-1);
        if(ans-k<0) cout<<"0"<<endl;
        else cout<<ans-k<<endl;
    }
    return 0;
}