[置顶] 归并排序，逆序数

写了个模板类，指针 a为被排序，b为tmp 返回逆序数，调用时候Merge.sort(a,b,n);

#define ll long long
template<class T>
class Merge {
public:
    T *a,*b;
    int n;
    ll num;
    ll sort(T *a1,T *b1,int n1){
        a=a1;b=b1;n=n1;
        num=0;
        divide(0,n-1);
        return num;
    }
    void divide(int l,int r){
        if(r>l){
            int mid=(l+r)>>1;
            divide(l,mid);
            divide(mid+1,r);
            merge(l,r);
        }
    }
    void merge(int l,int r){
        int mid=(l+r)>>1;
        int lpoint=l,rpoint=mid+1;
        int point=l;
        while(lpoint<=mid&&rpoint<=r){
            if(a[rpoint]<a[lpoint]){
                b[point++]=a[rpoint++];
                num+=(mid-lpoint+1);
            }
            else{
                b[point++]=a[lpoint++];
            }
        }
        while(lpoint<=mid){
            b[point++]=a[lpoint++];
        }
        while(rpoint<=r){
            b[point++]=a[rpoint++];
        }
        for(int i=l;i<=r;i++){
            a[i]=b[i];
        }
    }
};