//BFS solution
//Time: O(n), 2ms
//Space: O(h)
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null) return ans;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        int depth = 1;
        while(!q.isEmpty()) {
            int size = q.size();
            LinkedList<Integer> list = new LinkedList<>();
            for(int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                if(depth % 2 == 1) {
                    list.add(node.val);
                } else {
                    list.addFirst(node.val);
                }
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
            ans.add(list);
            depth++;
        }
        return ans;
    }
}
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */