Is it a Tree

    static Boolean is_it_a_tree(Integer node_count, ArrayList<Integer> edge_start, ArrayList<Integer> edge_end) {
        //Build the tree
        Map<Integer, ArrayList<Integer>> adj_list = new HashMap<>();
        for (int node = 0; node < node_count; node++) {
            adj_list.put(node, new ArrayList<Integer>());
        }
        for (int i = 0; i < edge_start.size(); i++) {
            adj_list.get(edge_start.get(i)).add(edge_end.get(i));
            adj_list.get(edge_end.get(i)).add(edge_start.get(i));
        }

        //Check whether there exist a cycle or not
        int[] visited = new int[node_count];
        int[] parent = new int[node_count];

        for (int node = 0; node < node_count; node++) {
            if (visited[node] == 0) {
                if (!bfs(node, visited, parent, adj_list)) {
                    return false;
                }
            }
        }

        return true;
    }

    static Boolean bfs(int node, int[] visited, int[] parent, Map<Integer, ArrayList<Integer>> adj_list) {
        Queue<Integer> q = new LinkedList<>();
        q.offer(node);
        visited[node] = 1;
        while (!q.isEmpty()) {
            int currNode = q.poll();
            for (int neighbor : adj_list.get(currNode)) {
                if (visited[neighbor] == 0) {
                    q.offer(neighbor);
                    visited[neighbor] = 1;
                    parent[neighbor] = currNode;
                } else {
                    if (parent[currNode] != neighbor) {
                        return false; //this means, it is a cycle
                    }
                }
            }
        }
        return true;
    }