297. Serialize and Deserialize Binary Tree

Problem

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

Clarification: The input/output format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

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

Example 1:

Input: root = [1,2,3,null,null,4,5]
Output: [1,2,3,null,null,4,5]

Example 2:

Input: root = []
Output: []

Constraints:

The number of nodes in the tree is in the range [0, 10^4].
-1000 <= Node.val <= 1000

Test Cases

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Codec:

    def serialize(self, root):
        """Encodes a tree to a single string.

        :type root: TreeNode
        :rtype: str
        """


    def deserialize(self, data):
        """Decodes your encoded data to tree.

        :type data: str
        :rtype: TreeNode
        """


# Your Codec object will be instantiated and called as such:
# ser = Codec()
# deser = Codec()
# ans = deser.deserialize(ser.serialize(root))

solution_test.py

import collections

import pytest

from solution import Codec
from utils import TreeNode


def build_tree(values: list[int | None]) -> TreeNode | None:
    virtual = TreeNode(None)
    queue = collections.deque()
    queue.append((virtual, 'left'))
    for val in values:
        node, branch = queue.popleft()
        if val is None:
            continue

        child = TreeNode(val)
        setattr(node, branch, child)
        queue.append((child, 'left'))
        queue.append((child, 'right'))

    return virtual.left


def trees_equal(root1: TreeNode | None, root2: TreeNode | None) -> bool:
    if root1 == root2:
        return True

    if root1.val != root2.val:
        return False

    return trees_equal(root1.left, root2.left) and trees_equal(root1.right, root2.right)


@pytest.mark.parametrize('values', [
    ([1,2,3,None,None,4,5]),
    ([]),

    ([1,2,3]),
    ([1,None,2,3]),
    ([5,4,7,3,None,2,None,-1,None,9]),
])
class Test:
    def test_solution(self, values):
        ser = Codec()
        deser = Codec()
        root = build_tree(values)
        data = ser.serialize(root)
        new_root = deser.deserialize(data)
        assert trees_equal(root, new_root)
        assert data == ser.serialize(new_root)

Thoughts

序列化的时候按照某个顺序遍历二叉树，依次打印访问到的节点的数据。可以用前序（pre-order，NLR）、中序（in-order，LNR）、后序（post-order，LRN）、层序（level-order）。

需要记录一定的分隔符（如对应位置的空节点），在反序列化的时候才能确定边界。

序列化时候几种遍历顺序都很容易实现，主要看反序列化的时候怎么比较方便。

LeetCode 用的是层序，并把空的子节点用 null 记录占位。层序在序列化和反序列化的时候，用队列辅助循环处理。

用前序遍历（NLR）。如果左右子节点为空，需要记占位符，直接用空字符串表示。节点值之间用逗号分割。序列化和反序列化的时候，都用栈辅助，避免递归。

比如上边 example 1 中的二叉树，会序列化为 "1,2,,,3,4,,,5,"。

Code

solution.py

from utils import TreeNode

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Codec:

    def serialize(self, root):
        """Encodes a tree to a single string.

        :type root: TreeNode
        :rtype: str
        """
        parts = []
        stack = []
        while root or stack:
            if root:
                stack.append(root)
                parts.append(str(root.val))
                root = root.left
            else:
                parts.append('')
                root = stack.pop().right

        return ','.join(parts)


    def deserialize(self, data):
        """Decodes your encoded data to tree.

        :type data: str
        :rtype: TreeNode
        """
        nodes = [TreeNode(int(s)) if s else None for s in data.split(',')]
        virtual = TreeNode(None)
        stack = [virtual]
        root = None
        for node in nodes:
            if root:
                stack.append(root)
                root.left = root = node
            else:
                stack.pop().right = root = node

        return virtual.right


# Your Codec object will be instantiated and called as such:
# ser = Codec()
# deser = Codec()
# ans = deser.deserialize(ser.serialize(root))