逆序对的数量

#include
using namespace std;

typedef long long LL;

const int N = 100005;
int n;
int a[N], tmp[N];

LL merge_sort(int l, int r)
{
    if(l >= r) return 0;
    
    int mid = l + r >> 1;
    
    LL res = merge_sort(l, mid) + merge_sort(mid + 1, r);
    
    int k = 0, i = l, j = mid + 1;
    while(i <= mid && j <= r)
    {
        if(a[i] <= a[j]) tmp[k++] = a[i++];
        else
        {
            tmp[k++] = a[j++];
            res += mid - i + 1;
        }
    }
    
    while(i <= mid) tmp[k++] = a[i++];
    while(j <= r) tmp[k++] = a[j++];
    
    for(int i = l, j = 0; i <= r; i++, j++) a[i] = tmp[j];
    
    return res;
}

int main()
{
    cin >> n;
    
    for(int i = 0; i < n; i++) scanf("%d", &a[i]);
    
    cout << merge_sort(0, n - 1) << endl;
    
    return 0;
}