A preorder tree traversal processes each node in a different order.

• Visit the node
• Process the left subtree preorder
• Process the right subtree preorder

The only difference is that we visit the root first, then go left, then right. The preorder output of the same binary tree will be:

26 14 9 21 79 53 35 99 87

A postorder tree traversal has this order:

• Process the left subtree postorder
• Process the right subtree postorder
• Visit the node

The prefix “post” refers to after, hence the location of visiting the node after the recursive calls. The printout of the same tree will be as follows:

9 21 14 35 53 87 99 79 26