Competitive-Coding/Centroid Decomposition at master · genalt/Competitive-Coding · GitHub

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

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

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

void decompose(int k, int par)
{
	nodes=0;
	dfs(k, k);
	int node=centroid(k, k);
	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: https://codeforces.com/contest/342/problem/E
//Solution 2: https://codeforces.com/contest/342/problem/E