[Leetcode] 103. Binary Tree Zigzag Level Order Traversal (JAVA)

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

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

The number of nodes in the tree is in the range [0, 2000].
-100 <= Node.val <= 100

My Code

//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 {
    private List<List<Integer>> ret;
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        ret = new ArrayList<>();
        helper(root, 0);
        return ret;
    }
    public void helper(TreeNode node, int depth) {
        if(node==null) return;
        if(ret.size()<=depth) ret.add(new ArrayList<Integer>());
        if(depth%2==0) ret.get(depth).add(node.val);
        else ret.get(depth).add(0, node.val);
        helper(node.left, depth+1);
        helper(node.right, depth+1);
    }
}

Comment

I'm in trouble using Tree node.