【算法】归并排序（可求逆序数或逆序对）

int L[100010];
int R[100010];

void merge(int p[],int l,int mid,int r){
    int len1 = mid - l;
    int len2 = r - mid;
    for(int i = 0;i < len1; i++){
        L[i] = p[l+i];
    }
    L[len1] = INF;
    for(int i = 0;i < len2; i++){
        R[i] = p[mid+i];
    }
    R[len2] = INF;
    
    int con1 = 0;
    int con2 = 0;
    for(int i = l;i < r; i++){
        if(L[con1] < R[con2]){
            p[i] = L[con1];
            con1++;
        }else if(L[con1] > R[con2]){
            p[i] = R[con2];
            con2++;
        }else{
            p[i] = L[con1];
            con1++;
        }
    }
}



void mergeSort(int p[],int l,int r){
    if(r - l > 1){
        int mid = (l+r)/2;
        mergeSort(p,l,mid);
        mergeSort(p,mid,r);
        
        merge(p,l,mid,r);
    }
}

l表示最左边的数的下标，r-1表示最右边的数的下标。