Problem: 2063C - Remove Exactly Two
Language: java

Solution 1 (runtime: 1421 ms):
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class RemoveExactlyTwo {

    public static int[][] dfs(int src, int par, List<List<Integer>> arr) {

        int[][] new_components = new int[3][2];

        int first_max = 0, second_max = 0;

        for (int v : arr.get(src)) {
            if (par == v) continue;

            int[][] components = dfs(v, src, arr);

            new_components[0][0] = Math.max(new_components[0][0], components[0][0]);
            new_components[0][1] = Math.max(new_components[0][1], components[0][1]);

            new_components[1][0] = Math.max(new_components[1][0], Math.max(components[1][0], components[1][1]));
            new_components[1][1] = Math.max(new_components[1][1], components[0][0] + arr.get(src).size());
            new_components[1][1] = Math.max(new_components[1][1], components[0][1] + arr.get(src).size() - 1);
            
            new_components[2][0] = Math.max(new_components[2][0], Math.max(components[2][0], components[2][1]));
            new_components[2][1] = Math.max(new_components[2][1], components[1][0] + arr.get(src).size());
            new_components[2][1] = Math.max(new_components[2][1], components[1][1] + arr.get(src).size() - 1);

            if (first_max < components[1][0] || first_max < components[1][1]) {
                second_max = first_max;
                first_max = Math.max(components[1][0], components[1][1]);
            } else if (second_max < components[1][0] || second_max < components[1][1]) {
                second_max = Math.max(components[1][0], components[1][1]);
            }
        }

        if (arr.get(src).isEmpty()) {

            new_components[0][0] = 1;
            new_components[0][1] = Integer.MIN_VALUE/2;

            new_components[1][0] = Integer.MIN_VALUE/2;
            new_components[1][1] = 0;

            new_components[2][0] = Integer.MIN_VALUE/2;
            new_components[2][1] = Integer.MIN_VALUE/2;
            
        } else {
            new_components[2][0] = Math.max(first_max + second_max, new_components[2][0]);
        }

        return new_components;
    }
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int t =  Integer.parseInt(br.readLine());

        while(t-- > 0) {

           int n = Integer.parseInt(br.readLine());
           
           int edges = n - 1;

           List<List<Integer>> arr = new ArrayList<>();

           for (int i = 0; i < n; i++) arr.add(new ArrayList<>());

           while (edges-- > 0) {
                int[] e = Arrays.stream(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();

                arr.get(e[0] - 1).add(e[1] - 1);
                arr.get(e[1] - 1).add(e[0] - 1);
           }

           int[][] components = dfs(0, -1, arr);

           System.out.println(Math.max(components[2][0], components[2][1]) - 1);

        }
    }
}

Solution 2 (runtime: 905 ms):

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class Main {
    static int m = 1000000007, M = 998244353;
    static long inf = (long) 1e18;

    static long timer = 0, max = -1;

    public static void main(String[] args) {
        FastReader next = new FastReader();
        int t = next.nextInt();
        l1:
        while (t-- > 0) {
            int n = next.nextInt();

            ArrayList<ArrayList<Integer>> lists = new ArrayList<>();
            int[] subTreeSize = new int[n + 1];
            int[] inTime = new int[n + 1], outTime = new int[n + 1];

            for (int i = 0; i <= n; i++) {
                lists.add(new ArrayList<>());
            }

            int[] neighbours = new int[n + 1];

            int p = 1;
            for (int i = 1; i < n; i++) {
                int v = next.nextInt();
                int u = next.nextInt();
                lists.get(v).add(u);
                lists.get(u).add(v);
            }
            int res = 0;
            int first = 0, second = 0;
            for (int i = 1; i <= n; i++) {
                if (lists.get(i).size() >= lists.get(first).size()) {
                    second = first;
                    first = i;
                } else {
                    if (lists.get(i).size() >= lists.get(second).size()) {
                        second = i;
                    }
                }
            }
//            System.out.println(lists);
            int max1= lists.get(first).size();
            int max2 = lists.get(second).size();
//            System.out.println(first  +" "+ second);
            ArrayList<ArrayList<Integer>> tem = deepCopy(lists);
            removeNodeEdges(tem, first);
//            System.out.println(tem);
            int f1 = findMaxDegree(tem);
            tem = deepCopy(lists);
            removeNodeEdges(tem, second);
//            System.out.println(tem);
            int f2 = findMaxDegree(tem);
//            ArrayList<Integer> first = lists.get(0), sec = lists.get(1);

            System.out.println(Math.max(max1 + f1 - 1, max2 + f2 - 1));
        }
    }

    private static ArrayList<ArrayList<Integer>> deepCopy(ArrayList<ArrayList<Integer>> original) {
        ArrayList<ArrayList<Integer>> copy = new ArrayList<>();
        for (ArrayList<Integer> list : original) {
            copy.add(new ArrayList<>(list));
        }
        return copy;
    }

    // Remove all edges connected to the given node
    private static void removeNodeEdges(ArrayList<ArrayList<Integer>> adjList, int node) {
        int ct = 0;
        for (ArrayList<Integer> neighbors : adjList) {
            if (ct == node){
                neighbors.clear();
                ct++;
                continue;
            }
            neighbors.removeIf(e -> e == node);
            ct++;
        }
    }

    // Find the maximum degree of any node in the adjacency list
    private static int findMaxDegree(ArrayList<ArrayList<Integer>> adjList) {
        int maxDegree = 0;
        for (ArrayList<Integer> neighbors : adjList) {
            maxDegree = Math.max(maxDegree, neighbors.size());
        }
        return maxDegree;
    }
    // Depth-First Search to count valid paths
    public static void dfs(int currentNode, int parent, ArrayList<ArrayList<Integer>> adjacencyList, int[] size, int[] in, int[] out) {
        size[currentNode] = 1;
        // Flag to check if current node is a leaf
        boolean isLeaf = true;

        // Traverse all connected nodes
        for (int neighbor : adjacencyList.get(currentNode)) {
            if (neighbor != parent) {
                isLeaf = false; // Not a leaf as it has a child
                dfs(neighbor, currentNode, adjacencyList, size, in, out);
                size[currentNode] += size[neighbor];
            }
        }
    }

    public static int[] shortestDistanceToLeaf(int n, ArrayList<ArrayList<Integer>> adj) {
        int[] dist = new int[n + 1];
        Arrays.fill(dist, -1);  // -1 indicates that the vertex has not been visited.
        Queue<Integer> queue = new LinkedList<>();

        // Special case: if the tree contains only one node, it is itself a leaf.
        if (n == 1) {
            dist[1] = 0;
            return dist;
        }

        // Identify all leaf nodes.
        // A vertex is a leaf if it has 0 or 1 neighbor.
        // (Typically, in a connected tree, leaves will have exactly 1 neighbor.)
        for (int i = 1; i <= n; i++) {
            if (adj.get(i).size() <= 1) {
                dist[i] = 0;
                queue.offer(i);
            }
        }

        // Multi-source BFS: propagate distances from all leaves simultaneously.
        while (!queue.isEmpty()) {
            int current = queue.poll();
            for (int neighbor : adj.get(current)) {
                if (dist[neighbor] == -1) {  // Not visited yet.
                    dist[neighbor] = dist[current] + 1;
                    queue.offer(neighbor);
                }
            }
        }
        return dist;
    }

    static void printTreeLevels(ArrayList<ArrayList<Integer>> graph, int root, int n) {
        Queue<Integer> queue = new LinkedList<>();
        boolean[] visited = new boolean[n];

        queue.add(root);
        visited[root] = true;

        int level = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            System.out.print("Level " + level + ": ");

            for (int i = 0; i < size; i++) {
                int node = queue.poll();
                System.out.print(node + " ");

                for (int neighbor : graph.get(node)) {
                    if (!visited[neighbor]) {
                        visited[neighbor] = true;
                        queue.add(neighbor);
                    }
                }
            }
            System.out.println();
            level++;
        }
    }

    private static boolean isAncestor(int x, int y, int[] in, int[] out) {
        return in[x] <= in[y] && out[x] >= out[y];
    }


    public static int lowerBound(ArrayList<Integer> arr, long target, int start, int end) {
        int left = start, right = end;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (arr.get(mid) < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        return left;
    }

    public 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 {
                if (st.hasMoreTokens()) {
                    str = st.nextToken("\n");
                } else {
                    str = br.readLine();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}