Pre-order Tree Traversal

Pre-order Tree Traversal is a depth-first way of visiting every node in a tree, where each node is visited before its children: visit the root, then traverse the left subtree, then the right subtree.

For a Binary Tree:

def pre_order(node):
    if node is None:
        return
    visit(node)
    pre_order(node.left)
    pre_order(node.right)

Since the root always comes first, pre-order traversal is useful for copying or serialising a tree. Compare with in-order (left, root, right) and post-order (left, right, root) traversal.