3
   / \
  9  20
    /  \
   15   7

[
  [15,7],
  [9,20],
  [3]
]

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) {
            return res;
        }

        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> levelVal = new LinkedList<>();
            while (size > 0) {
                TreeNode current = queue.poll();
                if (current.left != null) {
                    queue.add(current.left);
                }

                if (current.right != null) {
                    queue.add(current.right);
                }
                levelVal.add(current.val);
                size--;
            }
            res.add(0, levelVal);
        }

        return res;
    }
}

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) {
            return res;
        }
        helper(root, res, 0);
        return res;
    }

    public void helper(TreeNode root, List<List<Integer>> res, int depth) {
        if (root == null) {
            return;
        }

        if (depth == res.size()) {
            res.add(0, new LinkedList<>());
        }

        List<Integer> current = res.get(res.size() - depth - 1);
        helper(root.left, res, depth + 1);
        helper(root.right, res, depth + 1);
        current.add(root.val);
    }
}