ZigZag level order travsersal

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

ZigZagLevelOrder.java

package com.interviewbytes.trees;

import java.util.*;

public class ZigZagLevelOrder {

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {

        List<List<Integer>> list = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<>();
        if (root != null) queue.offer(root);

        boolean reverse = false;

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

            while (size > 0) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
                size--;
            }

            if (reverse) Collections.reverse(level);

            list.add(level);
            reverse = !reverse;
        }

        return list;
    }
}