最简代码

#include<bits/stdc++.h>
using namespace std;
long long n,cnt,num=10,x=9,a[10010];
int main(){
	cin>>n;
	if(n==1)cout<<9<<endl;
	a[1]=9;
	for(int i=2;i<=n;i++){a[i]=(7*x+num)%12345;x+=a[i];num=num*10%12345;}
	cout<<a[n];
	return 0;
}

#include<iostream> 
#include<iomanip> 
#include<set>
#include<string> 
#include<cstring>
#include<algorithm>
using namespace std;
const int N = 1e3 + 10;
int a[N] , b[N] , c = 9;
int main(){
	int n;
	cin >> n;
	a[1] = 8,b[1] = 1;
	for(int i = 2;i <= n;i++)
	{
		if(i == n) c = 8;
		a[i] = (a[i - 1] * 9 + b[i - 1] * 1) % 12345;
		b[i] = (a[i - 1] * 1 + b[i - 1] * 9) % 12345;
	}
	cout << a[n] << endl;
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
const int N = 1e3 + 10;
int a[N] , b[N] , c = 9;
int main(){
	int n;
	cin >> n;
	a[1] = 8,b[1] = 1;
	for(int i = 2;i <= n;i++)
	{
		if(i == n) c = 8;
		a[i] = (a[i - 1] * 9 + b[i - 1] * 1) % 12345;
		b[i] = (a[i - 1] * 1 + b[i - 1] * 9) % 12345;
	}
	cout << a[n] << endl;
	return 0;
}

#include <iostream>
#define int long long
using namespace std;
const int N=1e3+10;
const int mod=12345;
int odd[N],even[N];
signed main(){
	int n;
	cin>>n;
	even[1]=9;odd[1]=1;
	int t=9;
	for(int i=2;i<=n;i++){
		if(i==n)t=8;
		odd[i]=(t*odd[i-1]+even[i-1])%mod;
		even[i]=(1*odd[i-1]+t*even[i-1])%mod;
	}
	cout<<even[n]<<endl;
	return 0;
}