ENG  RUSTimus Online Judge
Online Judge
Online contests
About Online Judge
Frequently asked questions
Site news
Problem set
Submit solution
Judge status
Update your info
Authors ranklist
Current contest
Scheduled contests
Past contests
back to board

Discussion of Problem 2087. Trains

Posted by 👨‍💻tproger👨‍💻[GTGU] 6 Jun 2019 12:06
    For each vertex in tree find the nearest vertex from the way to root, in which we need to change direction. If such vertex doesn't exist, let it be -1. Let's call this vertex as bad[V], where V - some vertex in tree.
    To answer on query for vertex X, let's travel in "bad" array from X until we meet bad[X]=-1. Let's save visited vertices (excluding query vertex X) to some array (let's call this array "change"). Now it's quety obvious, that we need to change directions only in those visited vertices to "activate" path from root to leaf.     Before changing values in "bad" array for vertex from X to root, let's take some vertex V from "change" array. Before query there was a set of vertices which have the same "bad" vertex as a V (bad[V]=bad[V_1]=bad[V_2]=...=bad[V_n], V_i - vertex from the set), and now we need to change their "bad" value to V, because we change direction in vertex V.
    Finally, we need to set bad[V]=-1 to all vertices from X to root.
    To do all operations efficiently we can use heavy-light decomposiotion + segment tree. So, the total complexity would be O(N + Q*(log(N))^2), N - number of vertices in tree.

Edited by author 06.06.2019 17:11