The 14th Lanqiao Cup Competition Software Competition Final C/C++ University Group B Question A: Sub 2023

[Lanqiao Cup 2023 Country B] Sub 2023

Question A: Sub-2023

【Problem Description】

Xiaolan writes down $1$ to$For\; all\; integers\; between\; 2023$ , a number sequence is obtained:
$S=12345678910111213\cdots 20222023Xiaolan$
wants to knowSSHow many seed sequences in$S$$2023$？

Tip, the following are $3$ types of subsequences that meet the conditions (the numbers identified in square brackets are the numbers contained in the subsequence):

$1[\text{2}]34567891[\text{0}]111[\text{2}]1[\text{3}]14151617181920212223\cdots$
$1[\text{2}]34567891[\text{0}]111[\text{2}]131415161718192021222[\text{3}]\cdots$
$1[\text{2}]34567891[\text{0}]111213141516171819[\text{2}]021222[\text{3}]\cdots$
Note that the following is a subsequence that does not meet the conditions, although it contains$2$、$0$、$2$、$3$ four numbers, but the order is wrong:
$1[\text{2}]345678910111[\text{2}]131415161718192[\text{0}]21222[\text{3}]\cdots$

#include <bits/stdc++.h>
using namespace std;
typedef long long LL; 
int main()
{
    
    
	string s;
	for(int i=1;i<=2023;i++) s+=to_string(i);
	LL a=0;//2    de data 
	LL b=0;//20   de data
	LL c=0;//202  de data
	LL d=0;//2023 de data
	for(int i=0;i<s.size();i++) 
	{
    
    
		if(s[i]=='2') 
		{
    
    
			a++;
			c+=b;
		}
		if(s[i]=='0') b+=a;
		if(s[i]=='3') d+=c;
	}
	cout<<d;//5484660609
	
    return 0;
}