使用前缀和,记录有小于(大于)的值的数量。
C++ 代码
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
const int N = 100010;
int n;
int a[N], b[N], c[N];
int as[N]; // as[i]表示在A[]中有多少个数小于b[i]
int cs[N]; // cs[i]表示在C[]中有多少个数大于b[i]
int cnt[N], s[N];
int main()
{
scanf("%d", &n);
for (int i = 0; i < n; i ++ ) scanf("%d", &a[i]), a[i] ++ ;
for (int i = 0; i < n; i ++ ) scanf("%d", &b[i]), b[i] ++ ;
for (int i = 0; i < n; i ++ ) scanf("%d", &c[i]), c[i] ++ ;
// 求as[]
for (int i = 0; i < n; i ++ ) cnt[a[i]] ++ ;
for (int i = 1; i < N; i ++ ) s[i] = s[i - 1] + cnt[i]; // 求cnt[]的前缀和
for (int i = 0; i < n; i ++ ) as[i] = s[b[i] - 1];
// 求cs[]
memset(cnt, 0, sizeof cnt);
memset(s, 0, sizeof s);
for (int i = 0; i < n; i ++ ) cnt[c[i]] ++ ;
for (int i = 1; i < N; i ++ ) s[i] = s[i - 1] + cnt[i];
for (int i = 0; i < n; i ++ ) cs[i] = s[N - 1] - s[b[i]];
// 枚举每个b[i]
LL res = 0;
for (int i = 0; i < n; i ++ ) res += (LL)as[i] * cs[i];
cout << res << endl;
return 0;
}