Problem:
Score : 500 points

Problem Statement

#nck {
width: 30px;
height: auto;
}

There are N computers and N sockets in a one-dimensional world.
The coordinate of the i-th computer is a_i, and the coordinate of the i-th socket is b_i.
It is guaranteed that these 2N coordinates are pairwise distinct.

Snuke wants to connect each computer to a socket using a cable.
Each socket can be connected to only one computer.

In how many ways can he minimize the total length of the cables?
Compute the answer modulo 10^9+7.

Constraints

1 ≤ N ≤ 10^5

0 ≤ a_i, b_i ≤ 10^9

The coordinates are integers.

The coordinates are pairwise distinct.

Input

The input is given from Standard Input in the following format:

N
a_1
:
a_N
b_1
:
b_N

Output

Print the number of ways to minimize the total length of the cables, modulo 10^9+7.

Sample Input 1

2
0
10
20
30

Sample Output 1

2

There are two optimal connections: 0-20, 10-30 and 0-30, 10-20.
In both connections the total length of the cables is 40.

Sample Input 2

3
3
10
8
7
12
5

Sample Output 2

1

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		
		long mod = (long) 1e9 + 7;
		long[] frac = new long[N+1];
		frac[0] = 1;
		for(int i=1;i<=N;i++) frac[i] = (frac[i-1] * i) % mod;
		List<Pair> L = new List<Pair>();
		for(int i=0;i<N;i++){
			L.Add(new Pair(A[i], 1));
		}
		for(int i=0;i<N;i++){
			L.Add(new Pair(B[i], -1));
		}
		
		L.Sort((p, q) => p.Val.CompareTo(q.Val));
		long ans = 1;
		long tot = 0;
		long sig = 0;
		foreach(var p in L){
			sig += p.Sig;
			tot ++;
			if(sig == 0){
				ans *= frac[tot / 2];
				ans %= mod;
				tot = 0;
			}
		}
		
		Console.WriteLine(ans);
		
		
	}
	
	class Pair{
		public int Val, Sig;
		public Pair(int val, int sig){
			Val = val; Sig = sig; 
		}
	}
	
	
	int N;
	int[] A,B;
	public Sol(){
		N = ri();
		A = new int[N];
		for(int i=0;i<N;i++) A[i] = ri();
		B = new int[N];
		for(int i=0;i<N;i++) B[i] = ri();
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}