Day45.java

/* Binary Tree Zigzag Level Order Traversal */

import java.util.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {}

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();

        if (root == null) {
            return result;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean isLeftToRight = true;

        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            List<Integer> currentLevel = new ArrayList<>();

            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();

                // Add the node value based on the traversal direction
                if (isLeftToRight) {
                    currentLevel.add(node.val);
                } else {
                    currentLevel.add(0, node.val); // Insert at the beginning of the list
                }

                // Enqueue the child nodes
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }

            // Toggle the traversal direction
            isLeftToRight = !isLeftToRight;
            result.add(currentLevel);
        }

        return result;
    }
}

public class Day45 {
    public static void main(String[] args) {
        /*
                    3
                   / \
                  9   20
                     /  \
                    15   7
        */
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(9);
        root.right = new TreeNode(20);
        root.right.left = new TreeNode(15);
        root.right.right = new TreeNode(7);

        Solution solution = new Solution();
        List<List<Integer>> result = solution.zigzagLevelOrder(root);

        // Print the zigzag level order traversal
        for (List<Integer> level : result) {
            System.out.println(level);
        }
    }
}