Preorder Traversal — Root, Left, Right

Preorder prints (or visits) the root before its subtrees—useful for copying trees and building from traversal lists with markers.

Try this in Python

from __future__ import annotations
from dataclasses import dataclass


@dataclass
class TreeNode:
    val: int
    left: TreeNode | None = None
    right: TreeNode | None = None


def preorder(root: TreeNode | None) -> list[int]:
    out: list[int] = []

    def walk(n: TreeNode | None) -> None:
        if not n:
            return
        out.append(n.val)
        walk(n.left)
        walk(n.right)

    walk(root)
    return out

r = TreeNode(1, TreeNode(2, TreeNode(4)), TreeNode(3))
print(preorder(r))

Key takeaways

• Iterative preorder uses a stack pushing right before left so left is popped first.
• Know the three traversal orders by heart.
• Preorder + inorder can uniquely reconstruct if BST assumptions hold.