788逆序对的数量.cpp

#include <iostream>
using namespace std;
typedef long long LL;
const int N = 10 + 1e5;
int n;
int q[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 (q[i] <= q[j])
            tmp[k++] = q[i++];
        else
        {
            tmp[k++] = q[j++];
            res += mid - i + 1;
        }
    }
    while (i <= mid)
    {
        tmp[k++] = q[i++];
    }
    while (j <= r)
        tmp[k++] = q[j++];
    for (i = l, j = 0; i <= r; i++, j++)
    {
        q[i] = tmp[j];
    }
    return res;
}

int main()
{
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &q[i]);
    }
    printf("%lld", merge_sort(0, n - 1));
    return 0;
}