逆序对

#include<iostream>
#include<map>
#include<queue>
using namespace std;
typedef long long ll;
const int N=10000;
int e[N];
int tmp[N];
ll merge_sort(int l,int r)
{
    if(l>=r)
    return 0;
    int mid=l+r>>1;
    ll res=merge_sort(l,mid)+merge_sort(mid+1,r);
    int k=0,i=l,j=mid+1;
    while(i<=mid&&j<=r)
    {
        if(e[i]<=e[j]) tmp[k++]=e[i++];
        else
        {
            tmp[k++]=e[j++];
            res+=mid-i+1;
        }
    }
    while(i<=mid) tmp[k++]=e[i++];
    while(j<=r) tmp[k++]=e[j++];
    for(int i=l,j=0;i<=r;i++,j++)
    e[i]=tmp[j];
    return res;
}
int main()
{
    int t;
    cin>>t;
    for(int i=0;i<t;i++)
    cin>>e[i];
    cout<<merge_sort(0,t-1)<<endl;
    return 0;
}