按照覃老师的方法，竟然写对了！
class Solution {
private List<List<Integer>> ans;
public List<List<Integer>> levelOrder(TreeNode root) {
ans = new ArrayList<>();
_dfs(root, 0);
return ans;
}
private void _dfs(TreeNode node, int level) {
// terminator
if (node == null) return;
// process current logic
if (level > ans.size() - 1)
ans.add(new ArrayList<Integer>());
ans.get(level).add(node.val);
// drill down
_dfs(node.left, level + 1);
_dfs(node.right, level + 1);
// reverse state
}
}