0144. Binary Tree Preorder Traversal

https://leetcode.com/problems/binary-tree-preorder-traversal

Description

Given the root of a binary tree, return the preorder traversal of its nodes' values.

Example 1:

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

Example 2:

**Input:** root = []
**Output:** []

Example 3:

**Input:** root = [1]
**Output:** [1]

Example 4:

**Input:** root = [1,2]
**Output:** [1,2]

Example 5:

**Input:** root = [1,null,2]
**Output:** [1,2]

Constraints:

• The number of nodes in the tree is in the range [0, 100].

• -100 <= Node.val <= 100

Follow up: Recursive solution is trivial, could you do it iteratively?

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