C. Counting Orders

#include <bits/stdc++.h>

using namespace std;

const int Mod = 1e9 + 7;

int main() {
    ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);

    int testcase;
    cin >> testcase;
    while(testcase--) {
        int n;
        cin >> n;

        vector <int> a, b;
        for (int i = 0; i < n; i++) {
            int x;
            cin >> x;
            a.push_back(x);
        }

        for (int i = 0; i < n; i++) {
            int x;
            cin >> x;
            b.push_back(x);
        }

        sort(a.begin(), a.end());
        sort(b.rbegin(), b.rend());

        long long int ans = 1;
        for (int i = 0; i < n; i++) {
            int it = upper_bound(a.begin(), a.end(), b[i]) - a.begin();
            if (it == n) ans *= 0;
            ans *= (n - it - i);
            ans %= Mod;
        }

        cout << ans << '\n';
    }

    return 0;
}