Rt, probably summarize it (updated from time to time a little lower
1. General did not specify the root, beginning pick a root (1 point tools people)
2. Two $ dfs $, time to start something from pre-human tool, the second time (also from the tool began) to change the root.
3. Used for: just make sure a certain point of time is something, the answer is basically OK. Enumeration of each point can not afford the complexity, the father let his son's answers can be utilized.
4. merger within the statistical answer may be the molecular trees, outside the sub-tree, generally within the sub-tree During the first $ dfs $ in just got the answer, by changing the external sub-tree root, looking for his father to get the answer relations .