CLRS Exercise 12.1-4

12.1-4
Give recursive algorithms that perform preorder and postorder tree walks in $\Theta(n)$ time on a tree of $n$ nodes.

Preorder traversal:

PREORDER-TREE-WALK(x)
        if (x != NIL)
                print x.key
                PREORDER-TREE-WALK(x.left)
                PREORDER-TREE-WALK(x.right)

Postorder traversal:

POSTORDER-TREE-WALK(x)
        if (x != NIL)
                PREORDER-TREE-WALK(x.left)
                PREORDER-TREE-WALK(x.right)
                print x.key