13. DFS Preorder

Easy

Topics

Helpful tools to learn & understand

Watch key transitions, pointers, and variables update step-by-step.

Open

Read clean, documented implementations across multiple patterns.

View Code

Sketch diagrams on an infinite canvas and draft notes directly inline.

Draw

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

Output: [1, 2, 3]

Explanation: The preorder traversal visits the root (1), then the right subtree (2, 3) in that order.

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

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

Explanation: The preorder traversal visits the root, then the left subtree, and finally the right subtree.

Input: root = []

Output: []

Explanation: An empty tree results in an empty traversal.

Input: root = [1]

Output: [1]

Explanation: A single node tree simply returns the value of the root.

Time Complexity

O(n)

Space Complexity

O(h)