"each node is labeled from 1 to n", this is in the first sentence of the description and also shown in the examples, however after spending hours to debug, I found out that in the main() function, it is decrementing the node's label so that it is actually labeled from 0 to n-1.

I just want to point this out, hopefully it will be helpful. This is for C++.

int main() { ...... // read and set edges for (int i = 0; i < m; i++) { int u, v; cin >> u >> v; u--, v--; // add each edge to the graph graph.add_edge(u, v); } int startId; cin >> startId; startId--; // Find shortest reach from node s vector distances = graph.shortest_reach(startId);