A. tree
When do the exam outline reminiscent of ancient problem of memory.
Some expect the tree to go a few steps.
Obviously directly solving equations.
However, the complexity of $ O (qn ^ 3) $, using the properties of the tree to optimize it,
Dfs directly over the course of the solution out to be $ O (qnlogmod) $, which is the inverse of the log yuan.
However, only 20 points.
Each point of the pre-expected number of steps walked each son, went to his father's expectations a few steps.
Tree multiplier seeking lca, the prefix tree two processing functions and would be finished.