首先可以想到用前缀和求a中小于b的元素总数，求c中大于b的元素种数，然后求满足的三元组数
然后发现二分也能做，用STL也能AC

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N = 1e5 + 10;
int n;
int cnt[N],s[N];
int a[N],b[N],c[N];
int as[N],cs[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]++;
    for(int i = 0;i < n;++i)    cnt[a[i]]++;
    for(int i = 1;i < N;++i)    s[i] = s[i - 1] + cnt[i];
    for(int i = 0;i < n;++i)    as[i] = s[b[i] - 1];
    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]];
    LL res = 0;
    for(int i = 0;i < n;++i)    res += (LL)as[i] * cs[i];
    printf("%lld\n",res);
    return 0;
}

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N = 1e5 + 10;
int n;
int a[N],b[N],c[N];

int main(){
    scanf("%d",&n);
    for(int i = 0;i < n;++i)    scanf("%d",&a[i]);
    for(int i = 0;i < n;++i)    scanf("%d",&b[i]);
    for(int i = 0;i < n;++i)    scanf("%d",&c[i]);
    sort(a,a + n);
    sort(b,b + n);
    sort(c,c + n);
    LL res = 0;
    for(int i = 0;i < n;++i){
        int x = b[i];
        int left,right;
        int l = 0,r = n - 1;
        while(l < r){
            int mid = l + r + 1 >> 1;
            if(a[mid] < x)  l = mid;
            else    r = mid - 1;
        }
        left = l;
        if(a[left] >= x)   left = -1;
        l = 0,r = n - 1;
        while(l < r){
            int mid = l + r >> 1;
            if(c[mid] > x)  r = mid;
            else    l = mid + 1;
        }
        right = r;
        if(c[right] <= x)  right = n;
        res += (LL)(left + 1) * (n - right);
    }
    printf("%lld\n",res);
    return 0;
}

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N = 1e5 + 10;
int n;
int a[N],b[N],c[N];

int main(){
    scanf("%d",&n);
    for(int i = 0;i < n;++i)    scanf("%d",&a[i]);
    for(int i = 0;i < n;++i)    scanf("%d",&b[i]);
    for(int i = 0;i < n;++i)    scanf("%d",&c[i]);
    sort(a,a + n);
    sort(b,b + n);
    sort(c,c + n);
    LL res = 0;
    for(int i = 0;i < n;++i){
        LL x = lower_bound(a,a + n,b[i]) - a;
        LL y = n - (upper_bound(c,c + n,b[i]) - c);
        res += x * y;
    }
    printf("%lld\n",res);
    return 0;
}