788. 逆序对的数量

 

 

#include <iostream>
using namespace std;

const int N = 1e6 + 10;

int n;
int q[N], tmp[N];
long long cnt;

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

int main()
{
    cin >> n;
    for (int i = 0; i < n; ++ i)    cin >> q[i];
    
    merge_sort(q, 0, n - 1);
    
    cout << cnt << endl;
    
    return 0;
}