Submission #125124

#	Time	Username	Problem	Language	Result	Execution time	Memory
125124		NMAC427	Islands (IOI08_islands)	C++17	90 / 100	888 ms	131076 KiB

islands

// https://oj.uz/problem/view/IOI08_islands

#include <bits/stdc++.h>
#define ff first
#define ss second

using namespace std;
using ll = long long int;

int N;

vector<pair<int, int>> to;
vector<vector<int>> from;
vector<bool> inCycle;

pair<ll, ll> dfsSolve(int v) {
	ll max1 = 0;
	ll max2 = 0;

	ll maxInternal = 0; // Maximum internal path;

	for (const int child: from[v]) {
		if (inCycle[child]) continue;  // Check here because we want to be able to pass in a node of the cycle

		auto childDFS = dfsSolve(child);
		childDFS.ff += to[child].ss;

		if (childDFS.ff >= max1) {
			max2 = max1;
			max1 = childDFS.ff;
		} else if (childDFS.ff > max2) {
			max2 = childDFS.ff;
		}

		maxInternal = max(maxInternal, childDFS.ss);
	}

	maxInternal = max(maxInternal, max1 + max2);
	return {max1, maxInternal};
}

signed main() {

	#ifndef __APPLE__
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	#endif

	cin >> N;

	to.resize(N);
	from.resize(N);

	for (int i = 0; i < N; i++) {
		cin >> to[i].ff >> to[i].ss;
		to[i].ff -= 1;

		from[to[i].ff].push_back(i);
	}


	// Generate inCycle Vector
	inCycle.assign(N, false);
	vector<bool> visited (N, false);

	for (int i = 0; i < N; i++) {
		if (visited[i]) continue;

		vector<int> cycle;
		int next = i;

		while (true) {
			visited[next] = true;
			cycle.push_back(next);

			if (visited[to[next].ff]) {
				bool __inCycle = false;
				for (int v: cycle)  {
					if (to[next].ff == v) __inCycle = true;
					if (__inCycle) inCycle[v] = true;
				}

				break;
			} else {
				next = to[next].ff;
			}
		}
	}


	// Solve
	ll ans = 0;

	for (int i = 0; i < N; i++) {
		if (!inCycle[i]) continue;

		ll compAns = 0;

		vector<int> cycle;
		vector<ll> dfsV;
		vector<ll> prefixSum = {0};
		int next = i;

		do {
			auto dfs = dfsSolve(next);
			compAns = max(compAns, dfs.ss);  // Internal Path

			cycle.push_back(next);
			dfsV.push_back(dfs.ff);
			prefixSum.push_back(prefixSum.back() + to[next].ss);
			next = to[next].ff;
		} while (next != i);

		for (int v: cycle) {
			inCycle[v] = false;  // Don't visit same component twice
		}


		ll prefixFwd = dfsV[0];
		ll prefixBwd = dfsV[0];

		for(int i = 1; i < cycle.size(); i++) {
		 	compAns = max(compAns, dfsV[i] + prefixSum[i] + prefixFwd);
		 	compAns = max(compAns, dfsV[i] - prefixSum[i] + prefixBwd + prefixSum.back());
		 	
		 	prefixFwd = max(prefixFwd, dfsV[i] - prefixSum[i]);
		 	prefixBwd = max(prefixBwd, dfsV[i] + prefixSum[i]);
		}

		ans += compAns;
	}

	cout << ans << "\n";

	return 0;
}

Compilation message (stderr)

islands.cpp: In function 'int main()':
islands.cpp:122:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 1; i < cycle.size(); i++) {
                  ~~^~~~~~~~~~~~~~

• Subtask 1: 33 / 33
• Subtask 2: 3 / 3
• Subtask 3: 4 / 4
• Subtask 4: 10 / 10
• Subtask 5: 10 / 10
• Subtask 6: 10 / 10
• Subtask 7: 10 / 10
• Subtask 8: 10 / 10
• Subtask 9: 0 / 10