https://www.luogu.com.cn/problem/P1980

#include<bits/stdc++.h>
using namespace std;
long long n, x, ans;
int a = 0;
string s;
char c;
int main(){
	cin>>n>>x;
	c = x + '0';
	for(long long i = 0; i <= n; i++){
		long long xx = i;
		while(xx){
			s[a++] = (xx % 10 + '0');
			xx /= 10;
			
		}
	}
	for(int i = 0; i <= a; i++){
		if(s[i] == c) ans++;
	}
	cout<<ans;
	return 0;
}

为什么此代码的a++会导致RE