POJ 2299 Ultra-QuickSort | 归并排序

求逆序对个数

#include<cstdio>
#define N 500010
using namespace std;
typedef long long ll;
ll n,a[N],b[N],ans;
void Merge_sort(ll a[],ll b[],ll l,ll r)
{
    ll  mid=l+r>>1,p=l-1,i,j;
    if (l<r)
    {
	Merge_sort(a,b,l,mid),Merge_sort(a,b,mid+1,r);
	for (i=l,j=mid+1;i<=mid && j<=r;)
	    if (a[i]<a[j]) b[++p]=a[i++];
	    else b[++p]=a[j++],ans+=mid-i+1;
	while (i<=mid) b[++p]=a[i++];
	while (j<=r) b[++p]=a[j++];
	for (i=l;i<=r;i++)
	    a[i]=b[i];
    }
}
int main()
{
    while (scanf("%lld",&n)!=EOF && n)
    {
	ans=0;
	for (int i=1;i<=n;i++)
	    scanf("%lld",a+i);
	Merge_sort(a,b,1,n);
	printf("%lld\n",ans);
    }
    return 0;
}