[bug][python] serialize-and-deserialize-binary-tree

[asagege]

请在提交 bug 之前先搜索

• [X] 我已经搜索过 issues，没有发现相同的 bug。

出错的题目链接

https://leetcode.com/problems/serialize-and-deserialize-binary-tree/

报错信息

Runtime Error AttributeError: 'NoneType' object has no attribute 'append' sb.append(str(root.val)).append(self.SEP) Line 28 in _serialize (Solution.py) self._serialize(root, sb) Line 18 in serialize (Solution.py) data = ser.serialize(root) Line 66 in helper (Solution.py) ret = DriverSolution().helper( Line 83 in _driver (Solution.py) _driver() Line 95 in (Solution.py)

你是否愿意提交 PR 修复这个 bug？

• [ ] 我愿意！