Bug Report for serialize-and-deserialize-binary-tree

[slytherin-afk]

Problem Number,

Serialize and Deserialize Binary Tree

Bug Description

The current implementation of deserialize() incorrectly creates children for nodes that should not exist, resulting in an invalid binary tree structure. Specifically, it allows nodes to be attached under null parents, which violates the definition of a binary tree.

Surprisingly, my test cases are still passing, which suggests the issue is not being caught by typical serialization-deserialization checks.

For example, with the input:

[1,2,3,null,null,4,5,6,7]

My code constructs nodes 6 and 7 as children of what should be a null node — something that should not be possible in a valid binary tree. This highlights a flaw in the deserialization logic that assumes a complete binary tree structure and doesn't properly respect null entries.

Language Used for Code

Python/Python3

Code used for Submit/Run operation

class Codec:

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

        :type root: TreeNode
        :rtype: str
        """
        if not root:
            return ""

        q = deque([root])
        strl = []
        while q:
            n = q.popleft()

            if n != None:
                q.append(n.left)
                q.append(n.right)
                strl.append(str(n.val))
            else:
                strl.append("null")
        return ",".join(strl)

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

        :type data: str
        :rtype: TreeNode
        """
        if not data:
            return

        ndata = data.split(",")

        def dfs(pos):
            if pos >= len(ndata):
                return None

            node = TreeNode(ndata[pos])
            node.left = dfs(2 * pos + 1)
            node.right = dfs(2 * pos + 2)

            return node

        return dfs(0)

Expected behavior

A proper deserialization should respect "null" entries as missing nodes and avoid generating children under those positions and properly check if resulting binary tree matches the original one.

[slytherin-afk]

My code is making a binary tree in which null nodes have children and the test cases are passing but in correct testing it should fail. My code creates a binary tree in which null nodes have children, and while the test cases are passing, it should fail in the correct testing.