Binary Tree — At Most Two Children

Each node has left and right references (either may be None). This is the default model for most coding interviews.

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_vals(root: TreeNode | None) -> list[int]:
    if not root:
        return []
    return [root.val] + preorder_vals(root.left) + preorder_vals(root.right)

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

Key takeaways

• Recursion depth equals tree height in the worst case—mind stack overflow on deep skewed trees.
• Iterative traversals use explicit stacks or queues.
• Decide inclusive semantics for empty trees early.