Why I got Runtime Error on first test w/o vector reserve and got Accepted with reserve?

Without

With

The only difference is line 77 (if you open it in text editor).

Also, it passes test on local machine even without reserve statement.

ImplicitSegmentTree(int l, int r) {
	lb = l; rb = r;
	n = r-l+1;
	next = 0;
	// TODO Change
	// nodes.reserve(4831707);
	nodes.push_back(Node());
	root = &nodes[next++];
}