bfs

public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    if (root == null) {
        return result;
    }
    // write your code here
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    while(!queue.isEmpty()) {
        List<Integer> level = new ArrayList<>();
        int size = queue.size();
        for (int i = 0; i < size; i++){
            TreeNode node=queue.poll();
            level.add(node.val);
            if(node.left != null) {
                queue.offer(node.left);
            }
            if(node.right != null) {
                queue.offer(node.right);
            }
        }
        result.add(level);
    }
    return result;
}