Phonomenal Reviews

#include <bits/stdc++.h>
 
using namespace std;
 
const int MX = 1e5 + 5;
 
vector<int> g[MX];
int depth[MX];
bool use[MX];
int n, m, sol, node, mini, nodes;
 
void dfs1(int v, int parent) {
	nodes++;
 
	for (auto x : g[v]) {
		if (x == parent || !use[x]) continue;
		depth[x] = depth[v] + 1;
 
		if (depth[x] > mini) {
			mini = depth[v];
			node = x;
		}
		dfs1(x, v);
	}
}
 
void dfs(int v, int parent) {
 
	for (auto x : g[v]) {
		if (x == parent) continue;
		dfs(x, v);
		use[v] |= use[x];
	}
}
 
int main() {
	cin >> n >> m;
	int root;
	for (int i = 0; i < m; i++) {
		int cur;
		cin >> cur;
		use[cur] = true;
		root = cur;
	}
 
	for (int i = 0; i < n - 1; i++) {
		int u, v;
		cin >> u >> v;
		g[u].push_back(v);
		g[v].push_back(u);
	}
 
	dfs(root, -1);
 
	dfs1(root, -1);
 
	mini = 0;
	nodes = 0;
	memset(depth, 0, sizeof(depth));
	dfs1(node, -1);
 
	sol = ((nodes - 1) * 2) - depth[node];
 
	cout << sol << '\n';
}