I was trying this problem HOLI but I am not able to come up with a good solution. I was thinking of pairing up the leaf nodes of the longest path in the tree. After pairing up, I remove the two paired nodes and continue with the next longest path. But this would give a O(n^2) approach resulting TLE. Can somebody tell me an efficient approach for the solving problem ? I would be happy if somebody helps me in this. Thank you in advance.

The idea is the following:

Let's take a look at one edge of the tree. If this edge is removed, there are two sub-trees instead of the initial tree. Let's assume that one of these sub-trees contains x vertices. Then there're exactly y = n — x vertices in another sub-tree. There're at most 2 * min(x, y) paths going through this edge. So all we need to know is how many vertices there are in each of two sub-trees if this edge is removed. (For edge i, let it be x[i] and y[i]) It's clear that the answer is the sum for all edges: 2 * min(x[i], y[i]) * weight[i] (weight[i] — weight of the i-th edge).

There's only one thing left to do to solve the problem: calculate x[i] and y[i] for all edges quickly. It can be done with simple sub-tree dp(we need to know the number of vertices in a sub-tree). This solution works in O(N) and it's pretty simple to implement. Here's my code.

Thank you for the reply. Can you please explain how did you get that there're at most 2 * min(x, y) paths going through an edge ? I am not getting this part.

Let's assume that for i-th edge x[i] > y[i]. If more than y[i] paths go through this edge, there'll be y[i] houses for people to stay in and more than y[i] people. So they will have to share a house.

Yes, I got it. The problem really simplifies a lot after this deduction which is not trivial. Got it accepted with this approach. Thanks a lot for the help.

pigeonhole principle

hey wanted to know how u came up with the idea

I am unable to understand two things- 1.purpose of cnt array 2. what are we achieving using min(cnt[to],n-cnt[to])*2*w[v][i]

It may be a silly doubt but are we sure doing c = 2 * (x, n — x) * weight will give max? Will we always find a configuration as I m thinking what if due to max contribution c of some edge, max contribution of some other edge may reduce.

This approach is based on pigeon hole principle.

What he is asking is how to prove there always exists a configuration (or pairing) in which every edge gets traversed exactly $$$2 \cdot \min(x, n - x)$$$ times.

Does someone have a nice proof for this?

http://zobayer.blogspot.com/2014/01/spoj-holi.html

Got from above site contains awesome explanation w.r.t to pigeon hole principle --------------

`First observation is, in order to maximize cost, all edges will be used to travel around. So, if we can calculate how many times each edge will be used, we can calculate the cost.`