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 Python (3.4.3) (judge verdict: Accepted)

import sys
stdin = sys.stdin
 
sys.setrecursionlimit(10**5) 
 
def li(): return map(int, stdin.readline().split())
def li_(): return map(lambda x: int(x)-1, stdin.readline().split())
def lf(): return map(float, stdin.readline().split())
def ls(): return stdin.readline().split()
def ns(): return stdin.readline().rstrip()
def lc(): return list(ns())
def ni(): return int(stdin.readline())
def nf(): return float(stdin.readline())

MOD = 10**9+7

devices = []

n = ni()
for _ in range(n):
    a = ni()
    devices.append((a, 1))
    
for _ in range(n):
    b = ni()
    devices.append((b, -1))
    
devices.sort()

pc = [0,0]

ans = 1
for point, typ in devices:
    if typ == 1 and pc[1] > 0:
        ans = (ans * pc[1]) % MOD
        pc[1] -= 1
        
    elif typ == 1:
        pc[0] += 1
        
    elif typ == -1 and pc[0] > 0:
        ans = (ans * pc[0]) % MOD
        pc[0] -= 1
        
    elif typ == -1:
        pc[1] += 1
        
print(ans)