[GeeksForGeeks] Extract Leaves of a Binary Tree in a Doubly Linked List

 1 import java.util.LinkedList;
 2 import java.util.Queue;
 3 
 4 public class ExtractLeavesToDLL {
 5     private TreeNode head = null;
 6     private TreeNode tail = null;
 7     public void extractLeaveNodesToDLL(TreeNode root) {
 8         recursionHelper(root);
 9     }
10     private boolean recursionHelper(TreeNode node) {
11         if(node == null) {
12             return false;
13         }
14         else if(node.left == null && node.right == null) {
15             if(head == null) {
16                 head = node;
17                 tail = node;
18             }
19             else {
20                 tail.right = node;
21                 node.left = tail;
22                 tail = node;
23             }
24             return true;
25         }
26         if(recursionHelper(node.left)) {
27             node.left = null;
28         }
29         if(recursionHelper(node.right)) {
30             node.right = null;
31         }
32         return false;
33     }
34     private void traverseDLL(TreeNode head) {
35         TreeNode curr = head;
36         while(curr != null) {
37             String prevNode = curr.left == null ? "null" : Integer.toString(curr.left.key);
38             String nextNode = curr.right == null ? "null" : Integer.toString(curr.right.key);
39             System.out.println(curr.key + "‘s prev node is " + prevNode + " and its next node is " + nextNode);
40             curr = curr.right;
41         }
42     }
43     private void traverseBT(TreeNode root) {
44         Queue<TreeNode> queue = new LinkedList<TreeNode>();
45         queue.add(root);
46         
47         while(!queue.isEmpty()) {
48             TreeNode curr = queue.poll();
49             String leftNode = curr.left == null ? "null" : Integer.toString(curr.left.key);
50             String rightNode = curr.right == null ? "null" : Integer.toString(curr.right.key);
51             System.out.println(curr.key + "‘s left node is " + leftNode + " and its right node is " + rightNode);
52             if(curr.left != null) {
53                 queue.add(curr.left);
54             }
55             if(curr.right != null) {
56                 queue.add(curr.right);
57             }
58         }
59     }
60     public static void main(String[] args) {
61         TreeNode[] nodes = new TreeNode[10];
62         for(int i = 0; i < nodes.length; i++) {
63             nodes[i] = new TreeNode(i + 1);
64         }
65         nodes[0].left = nodes[1]; nodes[0].right = nodes[2];
66         nodes[1].left = nodes[3]; nodes[1].right = nodes[4];
67         nodes[2].right = nodes[5];
68         nodes[3].left = nodes[6]; nodes[3].right = nodes[7];
69         nodes[5].left = nodes[8]; nodes[5].right = nodes[9];
70         ExtractLeavesToDLL test = new ExtractLeavesToDLL();
71         test.extractLeaveNodesToDLL(nodes[0]);
72         
73         //check dll is correctly constructed 
74         test.traverseDLL(test.head);
75         //check binary tree is correctly modified.
76         test.traverseBT(nodes[0]);
77     }
78 }