LeetCode - Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its zigzag level order traversal as:
[
  [3],
  [20,9],
  [15,7]
]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> list = new ArrayList<List<Integer>>();
        if (root == null) {
            return list;
        }
        boolean fromLeft = true;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            fromLeft = !fromLeft;
            Queue<TreeNode> levelQueue = new LinkedList<>();
            List<Integer> levelList = new ArrayList<>();
            while(!queue.isEmpty()) {
                TreeNode node = queue.remove();
                levelList.add(node.val);
                if (fromLeft) {
                    if(node.left != null) {levelQueue.add(node.left);}
                    if(node.right != null) {levelQueue.add(node.right);}
                    
                }
                else{
                    if(node.right != null) {levelQueue.add(node.right);}
                    if(node.left != null) {levelQueue.add(node.left);}
                }

            }
             list.add(levelList);
            if (!levelQueue.isEmpty()) {
                queue = levelQueue;
            }
        }
        return list;
        
    }
}