题解

#include<bits/stdc++.h>
#define rb(a,b,c) for(int a=b;a<=c;++a)
#define rl(a,b,c) for(int a=b;a>=c;--a)
#define LL long long
#define IT iterator
#define PB push_back
#define II(a,b) make_pair(a,b)
#define FIR first
#define SEC second
#define FREO freopen("check.out","w",stdout)
#define rep(a,b) for(int a=0;a<b;++a)
#define SRAND mt19937 rng(chrono::steady_clock::now().time_since_epoch().count())
#define random(a) rng()%a
#define ALL(a) a.begin(),a.end()
#define POB pop_back
#define ff fflush(stdout)
#define fastio ios::sync_with_stdio(false)
#define check_min(a,b) a=min(a,b)
#define check_max(a,b) a=max(a,b)
using namespace std;
const int INF=0x3f3f3f3f;
typedef pair<int,int> mp;
int n;
const int MAXN=3003;
const int MOD=1e9+7;
int s[MAXN],t[MAXN],z[MAXN],c[MAXN][MAXN],dp[MAXN][MAXN][2];
int main(){
	scanf("%d",&n);
	c[0][0]=1;
	rb(i,1,n)
		rb(j,0,n){
			c[i][j]=c[i-1][j];
			if(j) c[i][j]+=c[i-1][j-1],c[i][j]%=MOD;
		}
	rb(i,1,n){
		scanf("%d",&s[i]);
	}
	rb(i,1,n){
		scanf("%d",&t[i]);
	}
	sort(s+1,s+1+n);
	sort(t+1,t+1+n);
	rb(i,1,n){
		z[i]=z[i-1];
		while(z[i]!=n&&s[z[i]+1]<=t[i]){
			z[i]++;
		}
	}
	dp[0][0][0]=1;
	rb(i,0,n-1){
		rb(j,0,z[i]) 
			rep(flag,2){
				rb(k,0,z[i+1]-z[i]){
					(dp[i+1][j+k][flag|(k!=(z[i+1]-z[i]))]+=1ll*dp[i][j][flag]*c[z[i+1]-z[i]][k]%MOD)%=MOD;
				}
			}
		rb(j,0,z[i+1]-1){
			dp[i+1][j][0]+=1ll*dp[i+1][j+1][0]*(j+1)%MOD;
			dp[i+1][j][0]%=MOD;
			dp[i+1][j][1]=1ll*dp[i+1][j+1][1]*(j+1)%MOD;
		}
		dp[i+1][z[i+1]][1]=0;
	}
	int rest=0;
		rest+=dp[n][0][0];
		rest%=MOD;
		rest+=dp[n][0][1];
		rest%=MOD;
	cout<<rest<<endl;
	return 0;
}