Here is my ac java solution. It just based on level order traverse method. But I have a question about my code. If `n % 2 == 0`

, then I should add the `queue.poll().val`

into the first position of the list. At the beginning, I used `addFirst()`

, but there was compile error, it said cannot find method `addFirst()`

, and used `offerFirst()`

return the same result. In the end, I used `add(0, queue.poll().val)`

, it passed. I want to know the reason for this compile error. Welcome to comment and judge!

```
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> res = new LinkedList<List<Integer>>();
Queue<TreeNode> queue = new LinkedList<TreeNode>();
if (root == null) return res;
queue.offer(root);
int n = 0;
while (!queue.isEmpty()) {
int level = queue.size();
n++;
List<Integer> list = new LinkedList<Integer>();
for (int i = 0; i < level; i++) {
if (queue.peek().left != null) queue.offer(queue.peek().left);
if (queue.peek().right != null) queue.offer(queue.peek().right);
if (n % 2 == 0) {
// right -> left
list.add(0, queue.poll().val);
}
else {
// left -> right
list.add(queue.poll().val);
}
}
res.add(list);
}
return res;
}
```