vaibhavmisra's blog

By vaibhavmisra, history, 6 weeks ago, In English

Hello everyone, I have been trying to find shortest path using dijkstra with set by storing indices instead of index-distance pairs with the help of a custom comparator. It is failing on a very large test case and I am having a difficult time debugging it. Where am I doing wrong? Any type of hint would be appreciated.

The Problem My solution

vector<ll> d;
 
struct comp {
	bool operator()(const int& a, const int& b) const {
		return d[a] < d[b];
	}
};
 
void solve() {
	int n, m; cin >> n >> m;
	vector<vector<pii>> graph(n + 1);
	set<int, comp> q;											
	rep(m) {
		int a, b, c; cin >> a >> b >> c;
		graph[a].push_back(make_pair(b, c));
		graph[b].push_back(make_pair(a, c));
	}
	vector<int> p(n + 1, -1);
	d.resize(n + 1, LLONG_MAX);
	d[1] = 0;
	q.insert(1);
	while(!q.empty()) {
		int node = *q.begin();
		q.erase(q.begin());
		for(auto adj : graph[node]) {
			if(d[adj.ff] > d[node] + adj.ss) {
				if(q.find(adj.ff) != q.end()) q.erase(adj.ff); 
				d[adj.ff] = d[node] + adj.ss;
				p[adj.ff] = node;
				q.insert(adj.ff);
			}	
		}
	}
	if(d[n] == LLONG_MAX) cout << -1 << endl;
	else {
		vector<int> path;
		while(n != -1) {
			path.push_back(n);
			n = p[n];
		}
		rep(path.size()) cout << path[path.size() - i - 1] << ' '; cout << endl;
	}
}
 
 
 
 
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»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Check the following priority-queue based solution.

158638653

»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

You have a problem in your custom comparator. Change it to

spoiler

Basically when you have two distinct nodes sharing the same distance only one of them can exist in the set at a given time using your old comparator implementation.