/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if(root == null) return List.of();
        List<List<Integer>> ans = new ArrayList<>();
        Queue<TreeNode> q = new ArrayDeque<>();
        q.add(root);
        while(!q.isEmpty()){
            int n = q.size();
            List<Integer> vals = new ArrayList<>(n);
            while(n -- > 0){
                TreeNode node = q.poll();
                vals.add(node.val);
                if(node.left != null) q.add(node.left);
                if(node.right != null) q.add(node.right);
            }
            ans.add(vals);
        }

        return ans;
    }
}