数组中的逆序对

class Solution {
public:
    int InversePairs(vector<int> data) {
        if(data.empty())
            return 0;
        int n=data.size();
        vector<int> copy(n);
        return Inverse(data,copy,0,n-1);
    }
    int Inverse(vector<int> &data,vector<int> &copy,int start,int end)
    {
        if(start==end)
        {
            return 0;
        }
        int mid=(start+end)>>1;
        int left=Inverse(data,copy,start,mid);
        int right=Inverse(data,copy,mid+1,end);
        int count=0;
        int ll=mid;
        int rr=end;
        int k=end;
        while(ll>=start&&rr>mid)
        {
            if(data[ll]>data[rr])
            {
                count+=rr-mid;
                copy[k--]=data[ll--];
            }
            else
            {
                copy[k--]=data[rr--];
            }
        }
        while(ll>=start)
            copy[k--]=data[ll--];
        while(rr>mid)
            copy[k--]=data[rr--];
        for(int i=start;i<=end;i++)
            data[i]=copy[i];
        return left+right+count;
    }
};