ZigZagTraversal.java

import java.util.*;

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

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

public class ZigZagTraversal {
    

    public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        boolean leftToRight = true;

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

            for (int i = 0; i < size; i++) {
                TreeNode current = queue.poll();

                if (leftToRight) {
                    level.add(current.val);
                } else {
                    level.add(0, current.val);
                }

                if (current.left != null) {
                    queue.offer(current.left);
                }
                if (current.right != null) {
                    queue.offer(current.right);
                }
            }

            result.add(level);
            leftToRight = !leftToRight;
        }

        return result;
    }

    public static TreeNode buildTree(String[] nodes) {
        if (nodes == null || nodes.length == 0) {
            return null;
        }

        TreeNode root = new TreeNode(Integer.parseInt(nodes[0]));
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        int i = 1;
        while (!queue.isEmpty() && i < nodes.length) {
            TreeNode current = queue.poll();

            if (!nodes[i].equals("N")) {
                current.left = new TreeNode(Integer.parseInt(nodes[i]));
                queue.add(current.left);
            }
            i++;

            if (i < nodes.length && !nodes[i].equals("N")) {
                current.right = new TreeNode(Integer.parseInt(nodes[i]));
                queue.add(current.right);
            }
            i++;
        }

        return root;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        scanner.close();
        String[] nodes = input.split(" ");
        TreeNode root = buildTree(nodes);

        List<List<Integer>> result = zigzagLevelOrder(root);
        for (List<Integer> level : result) {
            for (int val : level) {
                System.out.print(val + " ");
            }
        }
    }
}