[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.
 1 import java.util.ArrayList;
 2 import java.util.LinkedList;
 3 import java.util.Queue;
 4 
 5 public class NodeWithoutSibling {
 6     public ArrayList<Integer> getNodesWithoutSiblingLevelOrder (TreeNode root){
 7         if(root == null) {
 8             return null;
 9         }
10         ArrayList<Integer> list = new ArrayList<Integer>();
11         Queue<TreeNode> queue = new LinkedList<TreeNode>();
12         queue.add(root);
13         while(!queue.isEmpty()) {
14             TreeNode curr = queue.poll();
15             if(curr.left != null) {
16                 queue.add(curr.left);
17             }
18             if(curr.right != null) {
19                 queue.add(curr.right);
20             }
21             if(curr.left != null && curr.right == null) {
22                 list.add(curr.left.val);
23             }
24             if(curr.left == null && curr.right != null) {
25                 list.add(curr.right.val);
26             }
27         }
28         return list;
29     }
30     
31     public ArrayList<Integer> getNodesWithoutSiblingPreOrder1 (TreeNode root){
32         ArrayList<Integer> list = new ArrayList<Integer>();        
33         preOrderHelper1(root, list);
34         return list;
35     }
36     private void preOrderHelper1(TreeNode node, ArrayList<Integer> list) {
37         if(node == null) {
38             return;
39         }
40         if(node.left != null && node.right == null) {
41             list.add(node.left.val);
42         }
43         if(node.left == null && node.right != null) {
44             list.add(node.right.val);
45         }        
46         preOrderHelper1(node.left, list);
47         preOrderHelper1(node.right, list);
48     }
49     
50     public ArrayList<Integer> getNodesWithoutSiblingPreOrder2 (TreeNode root) {
51         ArrayList<Integer> list = new ArrayList<Integer>();        
52         preOrderHelper2(root, list);
53         return list;        
54     }
55     private void preOrderHelper2(TreeNode node, ArrayList<Integer> list) {
56         if(node == null) {
57             return;
58         }
59         if(node.left != null && node.right != null) {
60             preOrderHelper2(node.left, list);
61             preOrderHelper2(node.right, list);            
62         }
63         else if(node.left != null) {
64             list.add(node.left.val);
65             preOrderHelper2(node.left, list);            
66         }
67         else if(node.right != null) {
68             list.add(node.right.val);
69             preOrderHelper2(node.right, list);                
70         }
71     }
72     
73     public static void main(String[] args) {
74         TreeNode n1 = new TreeNode(1);
75         TreeNode n2 = new TreeNode(2);
76         TreeNode n3 = new TreeNode(3);
77         TreeNode n4 = new TreeNode(4);
78         TreeNode n5 = new TreeNode(5);
79         TreeNode n6 = new TreeNode(6);
80         n1.left = n2; n1.right = n3;
81         n2.right = n4; n3.left = n5; n5.left = n6;
82         NodeWithoutSibling test = new NodeWithoutSibling();
83         ArrayList<Integer> list = test.getNodesWithoutSiblingPreOrder2(n1);
84         for(int i = 0; i < list.size(); i++) {
85             System.out.println(list.get(i));
86         }
87     }
88 }