[GeeksForGeeks] Print all nodes that don't have sibling in a binary tree.

Given a binary tree,  get all nodes that don't have sibling node, excluding the root node. 

Level order and pre order solutions.

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;

public class NodeWithoutSibling {
    public ArrayList<Integer> getNodesWithoutSiblingLevelOrder (TreeNode root){
        if(root == null) {
            return null;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode curr = queue.poll();
            if(curr.left != null) {
                queue.add(curr.left);
            }
            if(curr.right != null) {
                queue.add(curr.right);
            }
            if(curr.left != null && curr.right == null) {
                list.add(curr.left.val);
            }
            if(curr.left == null && curr.right != null) {
                list.add(curr.right.val);
            }
        }
        return list;
    }
    
    public ArrayList<Integer> getNodesWithoutSiblingPreOrder1 (TreeNode root){
        ArrayList<Integer> list = new ArrayList<Integer>();        
        preOrderHelper1(root, list);
        return list;
    }
    private void preOrderHelper1(TreeNode node, ArrayList<Integer> list) {
        if(node == null) {
            return;
        }
        if(node.left != null && node.right == null) {
            list.add(node.left.val);
        }
        if(node.left == null && node.right != null) {
            list.add(node.right.val);
        }        
        preOrderHelper1(node.left, list);
        preOrderHelper1(node.right, list);
    }
    
    public ArrayList<Integer> getNodesWithoutSiblingPreOrder2 (TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();        
        preOrderHelper2(root, list);
        return list;        
    }
    private void preOrderHelper2(TreeNode node, ArrayList<Integer> list) {
        if(node == null) {
            return;
        }
        if(node.left != null && node.right != null) {
            preOrderHelper2(node.left, list);
            preOrderHelper2(node.right, list);            
        }
        else if(node.left != null) {
            list.add(node.left.val);
            preOrderHelper2(node.left, list);            
        }
        else if(node.right != null) {
            list.add(node.right.val);
            preOrderHelper2(node.right, list);                
        }
    }
    
    public static void main(String[] args) {
        TreeNode n1 = new TreeNode(1);
        TreeNode n2 = new TreeNode(2);
        TreeNode n3 = new TreeNode(3);
        TreeNode n4 = new TreeNode(4);
        TreeNode n5 = new TreeNode(5);
        TreeNode n6 = new TreeNode(6);
        n1.left = n2; n1.right = n3;
        n2.right = n4; n3.left = n5; n5.left = n6;
        NodeWithoutSibling test = new NodeWithoutSibling();
        ArrayList<Integer> list = test.getNodesWithoutSiblingPreOrder2(n1);
        for(int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }
    }
}