题目传送门

这道题很难，我不会

代码

 1 #include<iostream>
 2 using namespace std;
 3 const int N = 5e5+5;
 4 int q[N],tmp[N],n;
 5 long long ans = 0;
 6 void merge_sort(int q[],int l,int r)
 7 {
 8     if(l>=r)
 9         return;
10     int mid = l+r >> 1;
11     merge_sort(q,l,mid);
12     merge_sort(q,mid+1,r);
13     int i = l, j = mid + 1,k = 0;
14     while(i<=mid && j<=r)
15     {
16         if(q[i]<=q[j])  tmp[k++] = q[i++];
17         else    tmp[k++] = q[j++],ans += mid - i + 1;
18     }
19     while(i<=mid)
20         tmp[k++] = q[i++];
21     while(j<=r)
22         tmp[k++] = q[j++];
23     for(int i = l,j = 0; i<=r ;i++,j++)
24         q[i] = tmp[j];
25 }
26 int main()
27 {
28     cin>>n;
29     for(int i=0; i < n ;i++)
30         cin>>q[i];
31     merge_sort(q,0,n-1);
32     cout<<ans<<endl;
33 }