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

Concept Flow - Tree Traversal Postorder Left Right Root

Start at root

↓

Traverse left subtree

↓

Traverse right subtree

↓

Visit root node

↓

Done

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

Execution Sample

DSA Go

func postorder(node *Node) {
  if node == nil {
    return
  }
  postorder(node.Left)
  postorder(node.Right)
  fmt.Println(node.Value)
}

This code prints nodes of a binary tree in postorder: left, right, then root.

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	nil	Return (no node)	[]
5	Traverse right subtree	nil	Return (no node)	[]
6	Visit node	D	Print D	[D]
7	Traverse right subtree	E	Traverse left subtree	[D]
8	Traverse left subtree	nil	Return (no node)	[D]
9	Traverse right subtree	nil	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	nil	Return (no node)	[D, E, B]
15	Traverse right subtree	nil	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	nil	Return (no node)	[D, E, B, F]
19	Traverse right subtree	nil	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]

💡 All nodes visited in postorder: left, right, root.

Variable Tracker

Variable	Start	After Step 6	After Step 11	After Step 16	After Step 21	Final
Visited Nodes	[]	[D]	[D, E, B]	[D, E, B, F]	[D, E, B, F, G, C]	[D, E, B, F, G, C, A]
Current Node	A	B	B	C	C	A

Key Moments - 3 Insights

Why do we visit the root node last in postorder traversal?

Because postorder means left subtree first, then right subtree, and only then the root node is visited, as shown in steps 6, 11, 16, 21, and finally 22 in the execution_table.

What happens when the node is nil during traversal?

The function returns immediately without action, as seen in steps 4, 5, 8, 9, 14, 15, 18, and 19 where traversal hits nil nodes and returns.

How does the traversal know when to stop going deeper?

It stops when it reaches a nil node (no child), returning back to the previous call, as shown in the execution_table steps where nil nodes cause returns.

Visual Quiz - 3 Questions

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

A [D, E]

B [D, E, B]

C [D, E, B, F]

D [D, E, B, F, G]

💡 Hint

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

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

A Step 16

B Step 21

C Step 13

D Step 6

💡 Hint

Look for the 'Visit node' action with node 'F' in the execution_table.

If the left child of node 'B' was nil, how would the visited nodes change after step 6?

A [D]

B []

C [E]

D [B]

💡 Hint

Step 6 visits node 'D' which is left child of 'B'; if nil, it would skip to right child 'E'.

Concept Snapshot

Postorder traversal visits nodes in this order:
1. Left subtree
2. Right subtree
3. Root node

Use recursion to traverse left, then right, then print root.
Stops when node is nil (no child).
Common for deleting trees or postfix expressions.

Full Transcript

Postorder traversal means visiting the left subtree first, then the right subtree, and finally the root node. The provided Go code recursively calls postorder on the left child, then the right child, and prints the node's value last. The execution table shows step-by-step how the traversal moves through the tree nodes, returning immediately when it hits a nil node (no child). The visited nodes accumulate in the order D, E, B, F, G, C, A, which matches the postorder sequence. Key moments clarify why the root is visited last, how nil nodes cause returns, and how traversal stops going deeper. The visual quiz tests understanding of visited nodes at specific steps and effects of missing children. The snapshot summarizes the traversal order and usage.