AcWing 788. 逆序对的数量
原题链接
简单
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int N = 100010;
int n;
int q[N];
int w[N];
long long merge_sort(int l, int r)
{
if (l >= r) return 0;
long long res = 0;
int mid = (l + r) / 2;
res += merge_sort(l, mid) + merge_sort(mid + 1, r);
int i = l, j = mid + 1, k = 0;
while (i <= mid && j <= r)
if (q[i] <= q[j]) w[k ++ ] = q[i ++ ];
else
{
res += mid - i + 1;
w[k ++ ] = q[j ++ ];
}
while (i <= mid) w[k ++ ] = q[i ++ ];
while (j <= r) w[k ++ ] = q[j ++ ];
for (i = 0, j = l; i < k; i ++, j ++ ) q[j] = w[i];
return res;
}
int main()
{
cin >> n;
for (int i = 0; i < n; i ++ ) cin >> q[i];
cout << merge_sort(0, n - 1) << endl;
return 0;
}