Inversion

3 1
2 2 1
3 0
2 2 1

1
2

//归并排序 109MS	1820K
#include<stdio.h>
#include<string.h>
#define M 100007
#define ll __int64
ll a[M],c[M],ans;
void merge(ll a[],int first,int mid,int last,ll c[])
{
    int i=first,j=mid+1;
    int m=mid,n=last,k=0;
    while(i<=m||j<=n)
    {
        if(j>n||(i<=m&&a[i]<=a[j]))c[k++]=a[i++];
        else {c[k++]=a[j++];ans+=(m-i+1);}
    }
    for(i=0;i<k;i++)
        a[first+i]=c[i];
}
void merge_sort(ll a[],int first,int last,ll c[])
{
    if(first<last)
    {
        int mid=(first+last)>>1;
        merge_sort(a,first,mid,c);
        merge_sort(a,mid+1,last,c);
        merge(a,first,mid,last,c);
    }
}
int main()
{
    int n;
    ll k;
    while(scanf("%d%I64d",&n,&k)!=EOF)
    {
        memset(a,0,sizeof(a));
        memset(c,0,sizeof(c));
        ans=0;
        for(int i=0;i<n;i++)
            scanf("%I64d",&a[i]);
        merge_sort(a,0,n-1,c);
        if(ans-k<0)printf("0\n");
        else printf("%I64d\n",ans-k);
    }
    return 0;
}

//线段树 546MS	27296K
#include<stdio.h>
#include<string.h>
#include<algorithm>
#define M 500007
#define ll __int64
using namespace std;
int s[M],n;
struct Tree
{
    int l,r,mid;
    ll val;
}tree[M<<1];
struct sa
{
    int id;
    ll val;
}p[M*2];
int cmp(sa a,sa b)
{
    return a.val<b.val;
}
void build(int left,int right,int i)
{
    tree[i].l=left;tree[i].r=right;tree[i].mid=(left+right)>>1;tree[i].val=0;
    if(left==right){return;}
    build(left,tree[i].mid,i*2);
    build(tree[i].mid+1,right,i*2+1);
}
int query(int left,int right,int i)
{
    if(left>right)return 0;
    if(tree[i].l==left&&tree[i].r==right)return tree[i].val;
    if(right<=tree[i].mid)query(left,right,i*2);
    else if(left>tree[i].mid)query(left,right,i*2+1);
    else return query(left,tree[i].mid,2*i)+query(tree[i].mid+1,right,i*2+1);
}
void insert(int left,int i)
{
    tree[i].val++;
    if(tree[i].l==tree[i].r)return;
    if(left<=tree[i].mid)insert(left,2*i);
    else insert(left,2*i+1);
}
void discretization()
{
    int tmp=p[1].val,pos=1;
    for(int i=1;i<=n;i++)
        if(p[i].val!=tmp)tmp=p[i].val,p[i].val=++pos;
        else p[i].val=pos;
    for(int i=1;i<=n;i++)
        s[p[i].id]=p[i].val;
}
int main()
{
    __int64 k;
    while(scanf("%d%I64d",&n,&k)!=EOF)
    {
        ll ans=0;
        build(0,M,1);
        memset(s,0,sizeof(s));
        for(int i=1;i<=n;i++)
        {
            scanf("%I64d",&p[i].val);
            p[i].id=i;
        }
        sort(p+1,p+n+1,cmp);
        discretization();
        for(int i=1;i<=n;i++)
        {
                ans+=query(s[i]+1,n,1);
                insert(s[i],1);
        }
        if(ans-k<0)printf("0\n");
        else printf("%I64d\n",ans-k);
    }
    return 0;
}