LeetCode-Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {

        List<List<Integer>> res=new ArrayList<List<Integer>>();
        ArrayList<TreeNode> queue=new ArrayList<TreeNode>();
        if (root == null){
            return res;
        }
        queue.add(root);
        while(!queue.isEmpty()){
            List<Integer> values=new ArrayList<Integer>();
            ArrayList<TreeNode> subQueue=new ArrayList<TreeNode>();
            for(int i=0; i<queue.size(); i++){
                values.add(queue.get(i).val);
                if(queue.get(i).left != null){
                    subQueue.add(queue.get(i).left);
                }
                if(queue.get(i).right != null){
                    subQueue.add(queue.get(i).right);
                }
            }
            res.add(values);
            queue=subQueue;
        }
        return res;
    }
}

DFS:

/**
 * 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>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root == null) {
            return res;
        }
        helper(root, 0, res);
        return res;
    }
    
    private void helper (TreeNode node, int level, List<List<Integer>> list) {
        if (node == null) {
            return;
        }
        // add a new list for holding values of new level
        if (level == list.size()) {
            list.add(new ArrayList<Integer>());
        }
        // add values to it's correspodning level list
        list.get(level).add(node.val);
        helper(node.left, level+1, list);
        helper(node.right, level+1, list);
    }
}