求排列的逆序数<归并><C++>

程序

#include<cstdio>using namespace std;void gb_b(int p[],int a[],int i,int j,long long &ans){    int e=(i+j)/2+1,q=i,w=0;    do{        if(a[q]>a[e]) p[w++]=a[e++];        else if(a[q]==a[e]) p[w++]=a[q++],p[w++]=a[e++],ans+=e-(i+j)/2-2;             else p[w++]=a[q++],ans+=e-(i+j)/2-1;    }while(q<=(i+j)/2&&e<=j);    if(q<=(i+j)/2) for(int k=q;k<=(i+j)/2;k++) p[w++]=a[k],ans+=e-(i+j)/2-1;    if(e<=j) for(int k=e;k<=j;k++) p[w++]=a[k];    w=0;    for(int k=i;k<=j;k++) a[k]=p[w++];}void gb_g(int p[],int a[],int i,int j,long long &ans){    if(j-i==0) return;    gb_g(p,a,i,(i+j)/2,ans);    gb_g(p,a,(i+j)/2+1,j,ans);    gb_b(p,a,i,j,ans);}int main(){    long long ans=0;    int n;    scanf("%d",&n);    int a[n],p[n];    for(int i=0;i<=n-1;i++) scanf("%d",&a[i]);    gb_g(p,a,0,n-1,ans);    printf("%lld",ans);    return 0;}