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144-binary-tree-preorder-traversal.md

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

Example 1:

e1

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

Input: root = [] Output: [] Example 3:

Input: root = [1] Output: [1] Example 4:

e4

Input: root = [1,2] Output: [1,2] Example 5:

e5

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?