1688 求逆序对

#include<cstdio>
#include<iostream>
#define N 100100
using namespace std;
int a[N],b[N],n;
long long k;
void binary_chop(int l,int r){
    if(l==r) return ;
    int mid=(l+r>>1);
    binary_chop(l,mid);binary_chop(mid+1,r);
    int p=l,q=l,j=mid+1;
    while(p<=mid&&j<=r){
        if(a[p]>a[j]){
            k+=mid-p+1;//重点
            b[q++]=a[j++];
        }
        else{
            b[q++]=a[p++];
        }
    }
    while(p<=mid) b[q++]=a[p++];
    while(j<=r) b[q++]=a[j++];
    for(int i=l;i<=r;i++) a[i]=b[i];
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%d",a+i);
    }
    binary_chop(1,n);
    cout<<k<<endl;
    return 0;
}