Level-Order

Iterative solution

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();

        if (root == null) return res;

        q.add(root);
        while (!q.isEmpty()) {
            int levelSize = q.size();
            List<Integer> list = new ArrayList<>();
            for(int i = 0; i < levelSize; i++) {
                TreeNode node = q.poll();
                list.add(node.val);
                if (node.left != null) q.add(node.left);
                if (node.right != null) q.add(node.right);
            }

            res.add(list);
        }


        return res;
    }
}

Level-order

Recursive

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>(); 
        levelOrderHelper(root, res, 0);
        return res;
    }

    private void levelOrderHelper(TreeNode root, List<List<Integer>> res, int height) {
        if (root == null) return;

        if (res.size() <= height) {
            res.add(new ArrayList<Integer>());
        }

        res.get(height).add(root.val);
        if (root.left != null) levelOrderHelper(root.left, res, height + 1);
        if (root.right != null) levelOrderHelper(root.right, res, height + 1);

    }
}