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144. Binary Tree Preorder Traversal

Easy


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

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Example 1:

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

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

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Input: root = [1]
+Output: [1]
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Constraints:

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

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