Tree Traversal Postorder Left Right Root in DSA Typescript - Execution Trace

Concept Flow - Tree Traversal Postorder Left Right Root

Start at root node

↓

Traverse left subtree

↓

Traverse right subtree

↓

Visit current node (root)

↓

Back to parent node or end if root

Postorder traversal visits left subtree first, then right subtree, and finally the root node.

Execution Sample

DSA Typescript

function postorder(node) {
  if (!node) return;
  postorder(node.left);
  postorder(node.right);
  console.log(node.value);
}

This code visits nodes in postorder: left child, right child, then the node itself.

Execution Table

Step	Operation	Node Visited	Action	Tree State (Visited Nodes)
1	Start at root	A	Traverse left subtree	[]
2	Traverse left subtree	B	Traverse left subtree	[]
3	Traverse left subtree	D	Traverse left subtree	[]
4	Traverse left subtree	null	Return (no node)	[]
5	Traverse right subtree	null	Return (no node)	[]
6	Visit node	D	Print D	[D]
7	Traverse right subtree	E	Traverse left subtree	[D]
8	Traverse left subtree	null	Return (no node)	[D]
9	Traverse right subtree	null	Return (no node)	[D]
10	Visit node	E	Print E	[D, E]
11	Visit node	B	Print B	[D, E, B]
12	Traverse right subtree	C	Traverse left subtree	[D, E, B]
13	Traverse left subtree	F	Traverse left subtree	[D, E, B]
14	Traverse left subtree	null	Return (no node)	[D, E, B]
15	Traverse right subtree	null	Return (no node)	[D, E, B]
16	Visit node	F	Print F	[D, E, B, F]
17	Traverse right subtree	G	Traverse left subtree	[D, E, B, F]
18	Traverse left subtree	null	Return (no node)	[D, E, B, F]
19	Traverse right subtree	null	Return (no node)	[D, E, B, F]
20	Visit node	G	Print G	[D, E, B, F, G]
21	Visit node	C	Print C	[D, E, B, F, G, C]
22	Visit node	A	Print A	[D, E, B, F, G, C, A]
23	End	null	Traversal complete	[D, E, B, F, G, C, A]

💡 Traversal ends after visiting root node A and printing all nodes in postorder.

Variable Tracker

Variable	Start	After Step 6	After Step 11	After Step 16	After Step 21	Final
Current Node	A	D	B	F	C	null
Visited Nodes	[]	[D]	[D, E, B]	[D, E, B, F]	[D, E, B, F, G, C]	[D, E, B, F, G, C, A]
Call Stack Depth	1	3	2	3	2	0

Key Moments - 3 Insights

Why do we visit the current node only after traversing left and right subtrees?

Because postorder means Left, Right, Root order. The execution_table rows 6, 11, 16, 20, 21, and 22 show the node is printed only after both subtrees are fully traversed.

What happens when a node has no left or right child?

The traversal returns immediately (rows 4, 5, 8, 9, 14, 15, 18, 19) without printing, ensuring leaf nodes are visited correctly.

How does the call stack depth change during traversal?

It increases when going deeper into left/right children and decreases after visiting nodes, as shown in variable_tracker under Call Stack Depth.

Visual Quiz - 3 Questions

Look at the execution_table, what nodes have been visited after step 11?

A [D, E]

B [D]

C [D, E, B]

D [D, E, B, F]

💡 Hint

Check the 'Tree State (Visited Nodes)' column at step 11 in execution_table.

At which step does the traversal visit node 'C'?

A Step 21

B Step 20

C Step 22

D Step 16

💡 Hint

Look for 'Visit node' operation with node 'C' in execution_table.

If node 'B' had no right child, how would the visited nodes list change after step 11?

A [D, B, E]

B [D, B]

C [D, E, B]

D [B]

💡 Hint

Without right child 'E', node 'B' visits only left child 'D' before itself, see execution_table pattern.

Concept Snapshot

Postorder traversal visits nodes in Left → Right → Root order.
Use recursion: traverse left subtree, then right subtree, then visit node.
Useful for deleting trees or evaluating expression trees.
Print or process node only after children are done.
Handles empty (null) nodes by returning immediately.

Full Transcript

Postorder tree traversal means visiting the left subtree first, then the right subtree, and finally the root node. The code recursively calls itself on the left child, then the right child, and prints the node's value last. The execution table shows each step visiting nodes D, E, B, F, G, C, and finally A in that order. When a node is null, the function returns immediately without printing. The call stack grows as recursion goes deeper and shrinks as nodes are visited. This traversal is useful when you need to process children before their parent, such as deleting nodes or evaluating expressions.