Binary Tree Preorder Traversal

Input: root = [1,null,2,3]

Output: [1,2,3]

The tree has root 1, with right child 2, which has left child 3. Preorder visits: 1 (root first), then no left child, then 2, then 3 (left of 2).

Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]

Output: [1,2,4,5,6,7,3,8,9]

Full traversal of a complex tree. Visit root 1, then fully traverse left subtree (2,4,5,6,7), then right subtree (3,8,9).

Input: root = []

Output: []

An empty tree returns an empty list.

Input: root = [1]

Output: [1]

A single node tree returns just that node's value.

Problem