Problem: 2063C - Remove Exactly Two
Language: java

Solution 1 (runtime: 1968 ms):
import java.util.*;

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

        int t = in.nextInt();

        while (t-- > 0) {
            int n = in.nextInt();


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

            HashSet<Integer>[] s = new HashSet[n];

            for (int i = 0; i < n-1; i++) {
                int x = in.nextInt()-1;
                int y = in.nextInt()-1;

                if(arr[x] == null) {arr[x] = new ArrayList<>(); s[x] = new HashSet<>();}
                if(arr[y] == null) {arr[y] = new ArrayList<>(); s[y] = new HashSet<>();}

                arr[x].add(y); arr[y].add(x);
                s[x].add(y); s[y].add(x);
            }

            Comparator<int[]> comparator = (a, b) -> Integer.compare(b[0], a[0]);

            PriorityQueue<int[]> pq = new PriorityQueue<>(comparator);

//            pq.add(new int[]{n,0});

            HashSet<Integer> vis = new HashSet<>();

            int ans = dfs(pq, 0, vis, s, arr);

            System.out.println(ans);

        }



    }

    public static int dfs(PriorityQueue<int[]> pq, int m, HashSet<Integer> vis, HashSet<Integer>[] s, ArrayList<Integer>[] arr)
    {
        vis.add(m);
        int v = s[m].size();

        int max = 0;

        if(pq.isEmpty()){
            max = 0;
        }
        else if(s[m].contains(pq.peek()[1])) {
            int[] a = pq.poll();
            max = v + a[0] - 2;
            if(!pq.isEmpty()) max = Math.max( v + a[0] - 2, v + pq.peek()[0] - 1);
            pq.add(a);
        }
        else if(!s[m].contains(pq.peek()[1]))
        {
            max = Math.max(max, v + pq.peek()[0] - 1);
        }

        pq.add(new int[]{v, m});

        for (int i = 0; i < arr[m].size(); i++) {
            if(vis.contains(arr[m].get(i))) continue;
            max = Math.max(max, dfs(pq,arr[m].get(i), vis, s, arr));
        }

        return max;

    }



    /*          ─────────▄▄───────────────────▄▄──
                ──────────▀█───────────────────▀█─
                ──────────▄█───────────────────▄█─
                ──█████████▀───────────█████████▀─
                ───▄██████▄─────────────▄██████▄──
                ─▄██▀────▀██▄─────────▄██▀────▀██▄
                ─██────────██─────────██────────██
                ─██───██───██─────────██───██───██
                ─██────────██─────────██────────██
                ──██▄────▄██───────────██▄────▄██─
                ───▀██████▀─────────────▀██████▀──
                ──────────────────────────────────
                   ────────────────────────────
                    ─────────────────────────
                     ──────█████████████───
                     ──────────────────────
                     ──────────────────────
     */
}


Solution 2 (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;
        }
    }
}