二分

建议看看小弟在 => 数的范围 <= 发布的有关二分的题解, 搭配食用
https://www.acwing.com/solution/acwing/content/11358/

// package 第四讲枚举与模拟与排序;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;


public class Main {

    static BufferedReader br;
    static int N = (int) 1e5 + 10;
    static int n, A[], B[], C[];

    static void init() throws NumberFormatException, IOException {
        br = new BufferedReader(new InputStreamReader(System.in));
        A = new int[N];
        B = new int[N];
        C = new int[N];
        n = Integer.parseInt(br.readLine().trim());

        String[] ss = br.readLine().trim().split(" ");
        for (int i = 0; i < n; i++) {
            A[i + 1] = Integer.valueOf(ss[i]);
        }
        ss = br.readLine().trim().split(" ");
        for (int i = 0; i < n; i++) {
            B[i + 1] = Integer.valueOf(ss[i]);
        }
        ss = br.readLine().trim().split(" ");
        for (int i = 0; i < n; i++) {
            C[i + 1] = Integer.valueOf(ss[i]);
        }

    }

    public static void main(String[] args) throws NumberFormatException,
            IOException {
        init();
        long res = 0;
        // 1. 使用sort+二分优化
        // res += (A中小于B[j]的个数) * (C中大于B[j]的个数)

        Arrays.sort(A, 1, n + 1);
        Arrays.sort(B, 1, n + 1);
        Arrays.sort(C, 1, n + 1);

        for (int j = 1; j <= n; j++) {

            int a = 0, b = 0, x = B[j];

            // 查找a
            int l = 1, r = n;
            while (l < r) {
                int mid = l + r + 1 >> 1;
                if (A[mid] < x) {
                    l = mid;
                } else {
                    r = mid - 1;
                }
            }
            if (A[l] >= x) { // 二分很有可能没有找到答案,记得判断
                a = 0;
            } else {
                a = l;
            }

            // 查找b
            l = 1;
            r = n;
            while (l < r) {
                int mid = l + r >> 1;
                if (x < C[mid]) {
                    r = mid;
                } else {
                    l = mid + 1;
                }
            }
            if (C[l] <= x) {
                b = 0;
            } else {
                b = n - l + 1;
            }

            res += 1L * a * b; // 注意: 如果乘数不转long会爆

        }

        System.out.println(res);

    }
}