方法描述

排序+多指针遍历，复杂度O(n)

代码

#include<iostream>
#include<algorithm>
using namespace std;

const int N = 1e5 + 5;
int a[N];
int b[N];
int c[N];

int main()
{
    int n;
    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);
    long long res = 0;
    long long pa = 0, pb = 0, pc = 0;
    for (pb = 0; pb < n; ++pb)
    {
        while (pc < n && c[pc] <= b[pb])
        {
            ++pc;
        }
        while (pa < n && a[pa] < b[pb])
        {
            ++pa;
        }
        res += pa * (n - pc);
    }
    printf("%lld\n", res);
    return 0;
}