[GeeksForGeeks] Print leftmost and rightmost nodes at each level of a binary tree.

Given a Binary Tree, Print the corner nodes at each level. The node at the leftmost and the node at the rightmost.

For example, output for following is 15, 10, 20, 8, 25.

Solution. Level Order Traversal using queue.

Core idea:  Level order traversal always visit nodes of one level from left to right.

And we know the number of nodes at each level by pre-reading the size of the queue.

 1 import java.util.ArrayList;
 2 import java.util.LinkedList;
 3 import java.util.Queue;
 4
 5 class TreeNode {
 6     TreeNode left;
 7     TreeNode right;
 8     int val;
 9     TreeNode(int val){
10         this.left = null;
11         this.right = null;
12         this.val = val;
13     }
14 }
15 public class Solution {
16     public ArrayList<TreeNode> getLeftRightMostAtEachLevel(TreeNode root) {
17         ArrayList<TreeNode> result = new ArrayList<TreeNode>();
18         if(root == null){
19             return result;
20         }
21         Queue<TreeNode> queue = new LinkedList<TreeNode>();
22         queue.offer(root);
23         while(!queue.isEmpty()){
24             int size = queue.size();
25             for(int i = 0; i < size; i++){
26                 TreeNode curr = queue.poll();
27                 if(i == 0){
28                     result.add(curr);
29                 }
30                 if(i > 0 && i == size - 1){
31                     result.add(curr);
32                 }
33                 if(curr.left != null){
34                     queue.offer(curr.left);
35                 }
36                 if(curr.right != null){
37                     queue.offer(curr.right);
38                 }
39             }
40         }
41         return result;
42     }
43 }