# Binary Tree-Postorder Traversal – Non Recursive Approach

Objective: Given a binary tree, write a non recursive or iterative algorithm for postorder traversal.

Tree Traversals – Postorder

Example:

Earlier we have seen “What is postorder traversal and recursive algorithm for it“, In this article we will solve it with iterative/Non Recursive manner.

Approach:

• We have seen how we do inorder and preorder traversals without recursion using Stack, But post order traversal will be different and slightly more complex than other two. Reason is post order is non-tail recursive ( The statements execute after the recursive call).
• If you just observe here, postorder traversal is just reverse of preorder traversal (1 3 7 6 2 5 4 if we traverse the right node first and then left node.)
• So idea is follow the same technique as preorder traversal and instead of printing it push it to the another Stack so that they will come out in reverse order (LIFO).
• At the end just pop all the items from the second Stack and print it.

Pseudo Code:

1. Push root into Stack_One.
2. while(Stack_One is not empty)
1. Pop the node from Stack_One and push it into Stack_Two.
2. Push the left and right child nodes of popped node into Stack_One.
3. End Loop
4. Pop out all the nodes from Stack_Two and print it.

See the animated image below and code for more understanding.

Postorder traversal

Complete Code:

Output:

```4 5 2 6 7 3 1
4 5 2 6 7 3 1
```

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If you find anything incorrect or you feel that there is any better approach to solve the above problem, please write comment.
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### 2 Responses

1. naween0423 says:

Why cant we use a queue here then .. ???

2. lipsa patel says:

On line 20 instead of “while(s1.isEmpty() == false)” you can use “while(!s1.isEmpty())”
Same on line 35.

line 36: should be System.out.print(s2.pop() + ” “);

line 52: The call should be i.preorderIteration(root);

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