实质是用归并排序求逆序对
#include <iostream>
using namespace std;
typedef long long LL;
const int N = 5e6 + 10;
int cnt;
int q[N], tmp[N];
LL quick_sort(int l, int r)
{
if (l >= r) return 0;
int mid = l + r >> 1;
LL res = quick_sort(l, mid) + quick_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 (int i = l, j = 0; i <= r; i ++ , j ++ ) q[i] = tmp[j];
return res;
}
int main()
{
int n;
while(scanf("%d", &n), n)
{
for (int i = 0; i < n; i ++ ) cin >> q[i];
printf("%lld\n", quick_sort(0, n - 1));
}
return 0;
}
不应该是merge_sort吗?
哈哈对,题目是超快我就写quick了