使用归并排序来计算逆序数

#include<iostream>
using namespace std;

unsigned int L[100002],R[100002];
unsigned int num[200002];
unsigned long long nixushu=0;

void _Merge(int left, int mid, int right)
{
    int n1 = mid-left;
    int n2 = right-mid;

    for(int i=0;i<n1;i++)
        L[i] = num[left+i];

    for(int i=0;i<n2;i++)
        R[i] = num[mid+i];


    L[n1]=R[n2]=1000000003;
    int js1=0,js2=0;

    for(int i=left;i<right;i++)
    {
        if(L[js1]<=R[js2])
        {
            num[i] = L[js1];
            js1++;
        }
        else
        {
            num[i] = R[js2];
            js2++;
            nixushu += n1-js1;

        }
    }

}

void mergeSort(int left, int right)
{
    if(left+1>=right)
        return;

    int mid = (left+right)/2;
    mergeSort(left,mid);
    mergeSort(mid,right);
    _Merge(left, mid, right);

}


int main()
{
    int n;
    cin>>n;
    for(int i=0;i<n;i++)
        cin>>num[i];
    mergeSort(0,n);
    cout<<nixushu<<endl;
}