Centroid Decomposition.cpp

int nodes = 0;
int subtree[N], parentcentroid[N];
set<int> g[N];

void dfs(int u, int par)
{
	nodes++;
	subtree[u] = 1;
	for(auto &it:g[u])
	{
		if(it == par)
			continue;
		dfs(it, u);
		subtree[u] += subtree[it];
	}
}

int centroid(int u, int par)
{
	for(auto &it:g[u])
	{
		if(it == par)
			continue;
		if(subtree[u] > (nodes >> 1))
			return centroid(u, it);
	}
	return u;
}

void decompose(int u, int par)
{
	nodes = 0;
	dfs(u, u);
	int node = centroid(u, u);
	parentcentroid[node] = par;
	for(auto &it:g[node])
	{
		g[it].erase(node);
		decompose(it, node);
	}
}

//Problem 1: https://codeforces.com/contest/322/problem/E
//Solution 1: https://codeforces.com/contest/322/submission/45791742

//Problem 2 (Same as above, with colors being reverted): https://www.spoj.com/problems/QTREE5/

//Problem 3: https://codeforces.com/contest/342/problem/E