逆序对——洛谷——1908

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int N=40010;
int a[N],b[N],n,ans=0;
void gb(int l,int mid,int r){
    int cnt=l,h1=l,h2=mid+1;
    while(h1<=mid&&h2<=r){
        if(a[h1]<=a[h2])b[cnt++]=a[h1++];
        else{b[cnt++]=a[h2++];ans+=mid-h1+1;}
    }
    while(h1<=mid)b[cnt++]=a[h1++];
    while(h2<=r)b[cnt++]=a[h2++];
    for(int i=l;i<=r;i++)a[i]=b[i];
}
void gbsort(int l,int r){
    if(l==r)return;int mid=(l+r)>>1;
    gbsort(l,mid);gbsort(mid+1,r);gb(l,mid,r);
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)scanf("%d",&a[i]);
    gbsort(1,n);printf("%d\n",ans);
    return 0;
}