Hello CODEFORCES !

I am now up solving problems from Div3 and Educational CF rounds . Here I Feeling a difficulty .

In Div3 A, B, C are solvable for me . I sometimes solve D . But most time I fail . I see that the problem's rating too higher for
me . :(

In Educational round sometimes C is easy , sometimes C is too hard ( even B ) . So :( .

So I want to solve problems with a certain difficulty from certain type of contest . Example : I want to solve problems from Div3 Who are 1300 rated .

So I ask help from you .

Thanks in advance .

Sorry for disturbing you .

I was trying to solve this SPOJ problem , but failed .

Any suggestion ? Hint ? Tutorial ? Or solution ?

Waiting for positive reply .

int main()
{
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
        string s;
	cin >> s;
	map <char,set<int>>mp;
	int tot = s.size();
	vector<bool>a(10, 0);
	for(int i=0;i<tot;i++)
	{
		char x = s[i];
		mp[x].insert(i);
		a[x -'0'] = 1;
	}
	for(int i=0;i<10;i++)
	{
		if(a[i]==0)
		{
			printf("%d",i);
			return 0;
		}
	}
	for(int i=10;;i++)
	{
		string c = to_string(i);
		int len = c.size();
		char st = c[0];
		bool ok = 1;
		for(auto it : mp[st])
		{
			int start = it;
			if(start + len > tot) continue;
			string cur = s.substr(start,len);
			if(cur == c) ok = 0;
		}
		if(ok==1)
		{
			printf("%d",i);
			return 0;
		}
	}
}

Thanks in advance .