逆序对模板

//归并排序求逆序数对
#include<bits/stdc++.h>
using namespace std;
char buf[1<<20],*P1=buf,*P2=buf;
#define gc() (P1==P2&&(P2=(P1=buf)+fread(buf,1,1<<20,stdin),P1==P2)?EOF:*P1++)
#define TT template<class T>inline
TT bool read(T &x){
    x=0;char c=gc();bool f=0;
    while(c<48||c>57){if(c==EOF)return 0;f^=(c=='-'),c=gc();}
    while(47<c&&c<58)x=(x<<3)+(x<<1)+(c^48),c=gc();
    if(f)x=-x;return 1;
}
int a[500008],tmp[500008],cnt,n;
void merge(int l,int mid,int r){
    int i=l,j=mid,k=l;
    while(i<mid&&j<r)
        if(a[i]<=a[j])tmp[k++]=a[i++];
        else tmp[k++]=a[j++],cnt+=mid-i;
    while(i<mid)tmp[k++]=a[i++];
    while(j<r)tmp[k++]=a[j++];
    for(k=l;k<r;k++) a[k]=tmp[k];
}
void mergeSort(int l,int r){//[l,r)
    if(l+1>=r)return;
    int mid=(l+r)>>1;
    mergeSort(l,mid);
    mergeSort(mid,r);
    merge(l,mid,r);
}
int main() {
    while(read(n),n){
        for(int i=0;i<n;++i)read(a[i]);
        cnt=0;
        mergeSort(0,n);
        printf("%d
",cnt);
    }
    return 0;
}