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;
        
    }
}

 

/**
 * Definition for a binary tree node.
 * public 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>> res = new ArrayList<List<Integer>>();
        if (root == null) {
            return res;
        }
        List<TreeNode> list = new ArrayList<>();
        boolean odd = true;
        list.add(root);
        while(!list.isEmpty()) {
            List<TreeNode> levelList = new LinkedList<>();
            List<Integer> levelRes = new ArrayList<>();
            if (odd) {
                for(int i = 0; i < list.size(); i++) {
                    levelRes.add(list.get(i).val);
                    if(list.get(i).left != null) {
                        levelList.add(list.get(i).left);
                    } 
                    if(list.get(i).right != null) {
                        levelList.add(list.get(i).right);
                    } 
                }
            } 
            else{
                for(int i = list.size()-1; i >=0; i--) {
                    levelRes.add(list.get(i).val);
                    if(list.get(i).right != null) {
                        levelList.add(0, list.get(i).right);
                    }
                    if(list.get(i).left != null) {
                        levelList.add(0, list.get(i).left);
                    }
                }
            }
            odd = !odd;
            res.add(levelRes);
            list = levelList;
        }
        return res;
    }
}