Problem: 2063C - Remove Exactly Two
Language: java

Solution 1 (runtime: 968 ms):
import java.io.*;
import java.util.*;




public class HelpLancelot {
    


    static FastReader in = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);
    static long mod = 1_000_000_007;
    
    static class Pair{
        int a, b;
        Pair(int a, int b){ this.a =a; this.b = b;}
    }
    
    static int[] isPerm(int a[]){
        int n = a.length;
        int pres[] = new int[n+1];
        Arrays.fill(pres, -1);
        pres[0] = 1;
        for(int i=0; i<n; i++) pres[a[i]] = i;
        
        for(int i=1; i<=n; i++) {
            if(pres[i] == -1) {
                pres[0] = -1;
                break;
            }
        }
        
        return pres;
    }

    public static void main(String[] args) {

        int t = in.nextInt();
        while (t-- > 0) {
            int n = in.nextInt();
            ArrayList<HashSet<Integer>> adj = new ArrayList<>();
            for(int i=0; i<n; i++){
                adj.add(new HashSet<>());
            }
            
            int deg[] = new int[n];
            for(int i=1; i<=n-1; i++){
                int u = in.nextInt(); int v = in.nextInt();
                u--; v--;
                deg[u]++; deg[v]++;
                adj.get(v).add(u);
                adj.get(u).add(v);
            }
            
            ArrayList<Pair> pair = new ArrayList<>();
            for(int i=0; i<n; i++) pair.add(new Pair(i, deg[i]));
            
            Collections.sort(pair, (p,q) -> q.b - p.b);
            
            if(n == 2) out.println(0);
            else {
                Pair pair1 = pair.get(0);
                Pair pair2 = pair.get(1);
                
                
                int ans = 0;
                if(adj.get(pair1.a).contains(pair2.a)) ans = Math.max(ans, pair1.b + pair2.b - 2);
                else ans = Math.max(ans, pair1.b + pair2.b - 1);
                
                for(int i=2; i<n; i++){
                    Pair pair3 = pair.get(i);
                    if(adj.get(pair3.a).contains(pair2.a)) ans = Math.max(ans, pair3.b + pair2.b - 2);
                    else ans = Math.max(ans, pair3.b + pair2.b - 1);
                    
                    if(adj.get(pair1.a).contains(pair3.a)) ans = Math.max(ans, pair1.b + pair3.b - 2);
                    else ans = Math.max(ans, pair1.b + pair3.b - 1);
                }
                out.println(ans);
            }
            
            
        }
        out.close();
    }

    static void inArr(int a[], int s, int e) {
        for (int i = s; i <= e; i++) {
            a[i] = in.nextInt();
        }
    }

    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new
                    InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}

Solution 2 (runtime: 1843 ms):
// https://codeforces.com/problemset/problem/2063/C

import java.util.*;

public class Main
{
    private static void dfs_dp (int v, int p, final ArrayList<Integer>[] tree, int[] dp)
    {
        for (int to : tree[v])
        if (to != p)
        {
            dfs_dp(to, v, tree, dp);

            dp[v] = Math.max(dp[v], Math.max(dp[to], tree[to].size()));
        }
    }

    private static int dfs_ans (int v, int p, final ArrayList<Integer>[] tree, final int[] dp)
    {
        int ans = 0;

        for (int to : tree[v])
        if (to != p)
            ans = Math.max(ans, dfs_ans(to, v, tree, dp));

        for (int to : tree[v])
        if (to != p)
            ans = Math.max(ans, tree[v].size()-1 + Math.max(tree[to].size() - 1, dp[to]));
        
        int maxi = -1;

        for (int to : tree[v])
        if (to != p)
        {
            int cur = Math.max(tree[to].size(), dp[to]);

            if (maxi != -1)
                ans = Math.max(ans, maxi + cur - 1);
            
            maxi = Math.max(maxi, cur);
        }

        return ans;
    }
    
    private static void solve (Scanner in)
    {
        int n = in.nextInt();

        ArrayList<Integer>[] tree = new ArrayList[n];

        for (int i = 0; i < n; i++)
            tree[i] = new ArrayList<Integer>();

        for (int i = 1; i < n; i++)
        {
            int u = in.nextInt() - 1;
            int v = in.nextInt() - 1;

            tree[u].add(v);
            tree[v].add(u);
        }

        int[] dp = new int[n];

        for (int i = 0; i < n; i++)
            dp[i] = 0;

        dfs_dp(0, -1, tree, dp);
        
        int ans = dfs_ans(0, -1, tree, dp);

        System.out.println(ans);
    }

    public static void main(String[] args)
    {
        Scanner in = new Scanner(System.in);

        int t = in.nextInt();

        for (int test = 0; test < t; test++)
            solve(in);

        in.close();
    }
}