1+ class Node :
2+ def __init__ (self , val ):
3+ self .val = val
4+ self .right = None
5+ self .left = None
6+ 7+ def serialise (root : Node ) -> list :
8+ """
9+ Serialise a binary tree into a list using a pre-order traversal approach.
10+ The function uses a stack to traverse the tree iteratively and appends node
11+ values to a list. If a node is a leaf, it appends "None" to indicate the end
12+ of a branch.
13+
14+ Parameters:
15+ root (Node): The root node of the binary tree.
16+
17+ Returns:
18+ list: A list representing the serialised binary tree.
19+ """
20+ if not root :
21+ return None
22+ 23+ stack = [root ]
24+ stream = []
25+ while stack :
26+ current = stack .pop ()
27+ stream .append (current .val )
28+ 29+ # Append "None" to indicate the end of a branch
30+ if not current .right and not current .left :
31+ stream .append ("None" )
32+ 33+ # Push right child first so that left child is processed first
34+ if current .right :
35+ stack .append (current .right )
36+ 37+ if current .left :
38+ stack .append (current .left )
39+ return stream
40+ 41+ def deserialise (stream : list ) -> Node :
42+ """
43+ Deserialise a list back into a binary tree. The function reverses the list
44+ and uses a helper function to recursively reconstruct the tree.
45+
46+ Parameters:
47+ stream (list): The list representing the serialised binary tree.
48+
49+ Returns:
50+ Node: The root node of the deserialised binary tree.
51+ """
52+ stream .reverse ()
53+ root = deserialise_helper (stream )
54+ return root
55+ 56+ def deserialise_helper (stream : list ) -> Node :
57+ """
58+ Helper function to recursively deserialise the list into a binary tree.
59+
60+ Parameters:
61+ stream (list): The list representing the serialised binary tree.
62+
63+ Returns:
64+ Node: The current node being reconstructed.
65+ """
66+ node_val = stream .pop ()
67+ if isinstance (node_val , str ) and node_val == "None" :
68+ return None
69+ 70+ node = Node (node_val )
71+ node .left = deserialise_helper (stream )
72+ node .right = deserialise_helper (stream )
73+ 74+ return node
75+ 76+ # Approach and Reasoning
77+ # ----------------------
78+ # The serialisation function uses an iterative pre-order traversal to convert the binary tree into a list.
79+ # It uses a stack to manage the traversal order, ensuring that the left child is processed before the right child.
80+ # "None" is appended to the list to indicate the end of a branch, which helps in reconstructing the tree during deserialisation.
81+ 82+ # The deserialisation function reverses the list and uses a recursive helper function to reconstruct the tree.
83+ # The helper function pops elements from the list, creating nodes and recursively setting their left and right children.
84+ # The use of "None" in the list helps identify when a branch ends, allowing the function to return None for leaf children.
85+ 86+ # Complexity Analysis
87+ # -------------------
88+ # Time Complexity:
89+ # - Serialisation: O(n), where n is the number of nodes in the tree. Each node is visited once.
90+ # - Deserialisation: O(n), where n is the number of nodes in the tree. Each node is reconstructed once.
91+ 92+ # Space Complexity:
93+ # - Serialisation: O(n), where n is the number of nodes in the tree. The list stores each node's value and "None" for leaves.
94+ # - Deserialisation: O(h), where h is the height of the tree. This is due to the recursive call stack during reconstruction.
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