Submission #799665

#	Time	Username	Problem	Language	Result	Execution time	Memory
799665		GusterGoose27	Parking (CEOI22_parking)	C++17	100 / 100	113 ms	20776 KiB

Main

#include <bits/stdc++.h>

using namespace std;

const int MAXN = 2e5+5;
int n, m;
vector<int> config[MAXN];
int inv[2*MAXN]; // 2*i+0 for bottom 1 for top
bool vis[MAXN];

typedef pair<int, int> pii;

vector<pii> res;
set<int> avail;

int get_empty() {
	if (avail.empty()) {
		// assert(false);
		cout << -1 << endl;
		exit(0);
	}
	return *avail.begin();
}

void move(int a, int b) {
	assert(config[a].size());
	assert(config[b].size() < 2);
	assert(config[b].empty() || config[a].back() == (config[b].back()^1));

	res.push_back(pii(a+1, b+1));
	if (config[b].empty()) avail.erase(b);
	config[b].push_back(config[a].back());
	config[a].pop_back();
	if (config[a].empty()) avail.insert(a);
	inv[config[b].back()] = 2*b+config[b].size()-1;
}

void dfs(int cur, bool picky = 0) {
	int v = config[cur][0];
	int p = inv[v^1]/2;
	if (config[p].size() == 1) {
		move(p, cur);
		return;
	}
	if (inv[v^1]&1) {
		move(p, cur);
		return dfs(p, picky);
	}
	if (picky) return;
	// if we are going to be picky, then we might as well do it from the other direction
	int s = p;
	vector<pii> qu;
	while (config[s].size() == 2 && !(inv[config[s][1]^1] & 1)) {
		int nxt = inv[config[s][1]^1]/2;
		qu.push_back(pii(s, nxt));
		s = nxt;
	}
	if (config[s].size() == 2) {
		int e = get_empty();
		move(s, e);
		s = e;
	}
	for (int i = qu.size()-1; i >= 0; i--) move(qu[i].first, qu[i].second);
	move(p, cur);
	if (config[s].size() == 1) dfs(s, picky);
}

int main() {
	ios_base::sync_with_stdio(false); cin.tie(NULL);
	cin >> n >> m;
	for (int i = 0; i < m; i++) {
		int x, y; cin >> x >> y;
		if (x > 0) {
			config[i].push_back(2*(x-1)+vis[x-1]);
			vis[x-1] = 1;
		}
		if (y > 0) {
			config[i].push_back(2*(y-1)+vis[y-1]);
			vis[y-1] = 1;
		}
	}
	for (int i = 0; i < m; i++) {
		for (int j = 0; j < config[i].size(); j++) {
			inv[config[i][j]] = 2*i+j;
		}
		if (config[i].empty()) avail.insert(i);
	}
	// first step: combine anything that can be combined without extra space usage
	/*
	Clearly, this will only be possible if we have two endpoints that can just be simply connected
	(i.e)
	a
	b a
	c b
	d c
	d
	*/
	for (int i = 0; i < m; i++) {
		if (config[i].size() == 1) {
			dfs(i, 1);
		}
	}

	// now, for each single bottom, combine it (again, this frees space).
	// this will always take exactly 1 empty spot, so this is the best option
	for (int i = 0; i < m; i++) {
		if (config[i].size() == 1) {
			dfs(i);
		}
	}
	// nothing else will free up space, so order no longer matters

	// ERROR HERE DUE TO SUBTASK 2
	for (int i = 0; i < m; i++) {
		if (config[i].empty()) continue;
		assert(config[i].size() == 2);
		if (config[i][0] == (config[i][1]^1)) continue;
		if (!(inv[config[i][1]^1] & 1)) continue; // for any cycle that has two top nodes, we want to start from one of those
		int e = get_empty();
		move(i, e);
		dfs(i, 0);
	}
	for (int i = 0; i < m; i++) {
		if (config[i].empty()) continue;
		assert(config[i].size() == 2);
		if (config[i][0] == (config[i][1]^1)) continue;
		// if (!(inv[config[i][1]^1] & 1)) continue; // for any cycle that has two top nodes, we want to start from one of those
		int e = get_empty();
		move(i, e);
		dfs(i, 0);
	}
	for (int i = 0; i < m; i++) assert(config[i].empty() || (config[i].size() == 2 && (config[i][0]^1) == config[i][1]));
	cout << res.size() << '\n';
	for (pii p: res) {
		cout << p.first << ' ' << p.second << '\n';
	}
}

Compilation message (stderr)

Main.cpp: In function 'int main()':
Main.cpp:83:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   83 |   for (int j = 0; j < config[i].size(); j++) {
      |                   ~~^~~~~~~~~~~~~~~~~~

• Subtask 1: 10 / 10
• Subtask 2: 10 / 10
• Subtask 3: 25 / 25
• Subtask 4: 15 / 15
• Subtask 5: 25 / 25
• Subtask 6: 15 / 15