Serialize and Deserialize Binary Tree in DSA Typescript - Execution Trace

Concept Flow - Serialize and Deserialize Binary Tree

Start Serialization

↓

Visit Node

↓

Serialize Left

↓

Add Node Value or Null

↓

Repeat Until All Nodes

↓

Output Serialized String

↓

Start Deserialization

↓

Read Next Value

↓

If Null -> Return null

↓

Create Node

↓

Deserialize Left

↓

Return Node

↓

Repeat Until String Ends

Serialization visits each node in preorder, recording values or nulls; deserialization reads values to rebuild the tree recursively.

Execution Sample

DSA Typescript

function serialize(root: TreeNode | null): string {
  if (!root) return 'null,';
  return root.val + ',' + serialize(root.left) + serialize(root.right);
}

function deserialize(data: string): TreeNode | null {
  const nodes = data.split(',');
  function build(): TreeNode | null {
    const val = nodes.shift();
    if (val === 'null' || val === undefined || val === '') return null;
    const node = new TreeNode(parseInt(val));
    node.left = build();
    node.right = build();
    return node;
  }
  return build();
}

This code converts a binary tree to a string and back using preorder traversal with 'null' placeholders.

Execution Table

Step	Operation	Node Visited	Pointer Changes	Serialized String / Tree State
1	Serialize root node	1	root	Serialized: '1,'
2	Serialize left child	2	root.left	Serialized: '1,2,'
3	Serialize left.left (null)	null	-	Serialized: '1,2,null,'
4	Serialize left.right (null)	null	-	Serialized: '1,2,null,null,'
5	Serialize right child	3	root.right	Serialized: '1,2,null,null,3,'
6	Serialize right.left (null)	null	-	Serialized: '1,2,null,null,3,null,'
7	Serialize right.right (null)	null	-	Serialized: '1,2,null,null,3,null,null,'
8	Deserialization start	-	-	Input: '1,2,null,null,3,null,null,'
9	Create root node	1	root	Tree: 1
10	Create left child	2	root.left	Tree: 1 -> 2 (left)
11	Left.left is null	null	-	Tree unchanged
12	Left.right is null	null	-	Tree unchanged
13	Create right child	3	root.right	Tree: 1 -> 3 (right)
14	Right.left is null	null	-	Tree unchanged
15	Right.right is null	null	-	Tree unchanged
16	Deserialization complete	-	-	Tree rebuilt: 1 -> 2 (left), 3 (right)

💡 All nodes visited and serialized; all values consumed and tree rebuilt.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
Serialized String	''	'1,'	'1,2,'	'1,2,null,'	'1,2,null,null,'	'1,2,null,null,3,'	'1,2,null,null,3,null,'	'1,2,null,null,3,null,null,'
Deserialization nodes array	['1','2','null','null','3','null','null','']	['2','null','null','3','null','null','']	['null','null','3','null','null','']	['null','3','null','null','']	['3','null','null','']	['null','null','']	['null','']	['']
Current Node	null	1	2	null	null	3	null	null

Key Moments - 3 Insights

Why do we add 'null' for empty children during serialization?

Adding 'null' placeholders ensures the tree structure is preserved exactly, as shown in steps 3, 4, 6, and 7 in the execution_table where nulls mark missing children.

How does deserialization know when to stop creating nodes?

Deserialization reads 'null' values to return null nodes, stopping recursion as seen in steps 11, 12, 14, and 15 where 'null' causes return without creating nodes.

Why do we use preorder traversal for serialization?

Preorder visits root before children, allowing deserialization to reconstruct the tree in the same order, matching the sequence in the serialized string from steps 1 to 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what is the serialized string after serializing the right child?

A '1,2,null,null,3,'

B '1,2,null,null,'

C '1,2,null,null,3,null,'

D '1,2,null,null,3,null,null,'

💡 Hint

Check the Serialized String column at step 5 in execution_table.

At which step does deserialization create the left child node?

A Step 9

B Step 10

C Step 13

D Step 11

💡 Hint

Look at the Operation and Node Visited columns in execution_table for deserialization steps.

If we remove 'null' placeholders during serialization, what will happen during deserialization?

A Deserialization will work correctly without changes

B The serialized string will be longer

C Deserialization will fail to rebuild the exact tree structure

D The tree will have extra nodes

💡 Hint

Refer to key_moments about why 'null' placeholders are needed.

Concept Snapshot

Serialize: preorder traversal, record node values and 'null' for empty children.
Deserialize: read values in order, create nodes or nulls recursively.
'null' placeholders keep tree shape exact.
Preorder ensures root is first, aiding reconstruction.
Use split and shift on string for deserialization.
Simple, recursive, and reversible process.

Full Transcript

Serialization of a binary tree uses preorder traversal to visit each node. For each node, its value is recorded followed by serializing its left and right children. If a child is missing, 'null' is recorded to keep the tree shape. This process creates a string representing the tree structure. Deserialization reads this string from left to right, creating nodes or nulls recursively in preorder. This rebuilds the original tree exactly. The execution table shows each step of serialization and deserialization, tracking the serialized string and tree state. Key moments clarify why 'null' placeholders are essential and why preorder traversal is used. The visual quiz tests understanding of these steps and their effects. This method is simple, reliable, and widely used for tree data transfer.