```
public List<List<Integer>> levelOrderBottom(TreeNode root) {
// first intuitive idea is just the same as level order and still use ArrayList
// then reverse it, but we will have extra O(n) time complexity
// Thanks to the discussion, they suggested to add at head.
// then we should implement result as LinkedList not ArrayList anymore, because
// insert at ArrayList's head is so expensive.
List<List<Integer>> res = new LinkedList<>();
Queue<TreeNode> q = new LinkedList<>();
if (root == null) {
return res;
}
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
List<Integer> cur = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode node = q.poll();
cur.add(node.val);
if (node.left != null) {
q.offer(node.left);
}
if (node.right != null) {
q.offer(node.right);
}
}
res.add(0, cur);
}
return res;
}
```