USE DFS

/**
 * 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>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        helper(root, 0, ans);
        return ans;
    }
    public void helper(TreeNode root, int level, List<List<Integer>> ans) {
        if (root == null) {
            return;
        }
        if (level >= ans.size()) {
            ans.add(new ArrayList<>());
        }
        ans.get(level).add(root.val);
        helper(root.left, level + 1, ans);
        helper(root.right, level + 1, ans);
    }
}