题目描述

blablabla

样例

#include<iostream>
using namespace std;

typedef long long LL;

const int N = 100010;
int q[N],tmp[N];

LL merge_sort(int q[], int l, int r)
{
    if(l >= r) return 0;

    int mid = l + r >> 1, i = l, j = mid + 1, k = 0;
    LL res = merge_sort(q, l, mid) + merge_sort(q, mid+1, r);

    while(i <= mid && j <= r)
    {
        if(q[i] <= q[j]) tmp[k++] = q[i++];
        else
        {
            res += 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 res;

}

int main()
{
    int n;
    scanf("%d", &n);

    for(int i = 0; i < n; i++) scanf("%d",&q[i]);

    LL res = merge_sort(q, 0, n-1);

    cout << res << endl;

    return 0;
}