[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];

        }

    }

};