advent-of-code/2023/4/main-2.cpp

#include <bits/stdc++.h>
using namespace std;

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);

	vector<int> counts, copies;

	string line;
	while (getline(cin, line) && !line.empty()) {
		int start = line.find(':');
		int bar = line.find('|');
		string left = line.substr(start + 2, bar - start - 3);
		string right = line.substr(bar + 2);
		istringstream lss(left), rss(right);
		set<int> ls;
		int count = 0;
		while (!lss.eof()) {
			int n;
			lss >> n;
			ls.insert(n);
		}
		while (!rss.eof()) {
			int n;
			rss >> n;
			if (ls.find(n) != ls.end())
				count++;
		}
		counts.push_back(count);
		copies.push_back(1);
	}
	int ret = 0;
	for (int cn = 0; cn < counts.size(); cn++) {
		ret += copies[cn];
		int n = cn;
		for (int i = 0; i < counts[cn]; i++) {
			if (++n == copies.size())
				break;
			copies[n] += copies[cn];
		}
	}
	cout << ret << '\n';
}