Last contest,I failed system test on D,that I didn't become an IGM.

But I don't know why my D failed system test,I submitted the same code $$$3$$$ times today,they all get Accepted.See 72380842 72380904 72381047

I think it's the problem of randomized integers,but I don't know why.Please help me!

Here's my solution to D: Let's Maintain candidate roots,at the beginning,every vertex is a candidate root.In each query,we randomly choose two candidate roots that are not adjacent ans not equal.Let's denote them $$$x$$$,$$$y$$$,then we ask $$$x$$$,$$$y$$$'s lca,denote it $$$w$$$，then for every $$$w$$$'s subtree that contains $$$x$$$ or $$$y$$$,all of the vertex in it can be removed from candidate root.It can be shown that in each query,we remove at least two candidate roots.All the vertices left form a subtree of the original tree.There's a special case on $$$n=2$$$,just ask these two vertices.

Here's my submission:72324004

Sorry for my poor English.