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# Post-order traversal

In computer science, post-order traversal is used in data structures, and specifically, trees and binary trees.

Programs that utilize tree structures need to process nodes in a tree (represented as circles in below diagram). Nodes contain information about an object. For now, let's assume each node contains a letter.

Post-order traversal is a type of tree traversal algorithm. Post-order occurs when the root is postponed until its two subtrees are processed.

### Steps to post-order traversal

Given a non-empty tree,

Given a binary tree PY:

The order would go D,G,E,B,F,C,A

An example of PostOrder in C++

```template
int postorder_print(const binary_tree_nodes* ptr)
// ptr is a pointer to a node in a binary tree OR null
// meaning empty tree.
{
if (ptr != NULL)
{
postorder_print( ptr->left() );
postorder_print( ptr->right() );
std::cout << ptr->data() << std::endl;
}
return 0;
}
```
The same example in
Haskell might look like

```data Tree a = ET | Node(a, Tree a, Tree a)
postorder :: Tree a -> [a]
postorder ET = []
postorder (Node (x, left,right)) = (postorder left) ++ (postorder right) ++ [x]
```
Compare: Pre-order traversal, Inorder traversal