144. Binary Tree Preorder Traversal
[https://leetcode.com/problems/binary-tree-preorder-traversal/?tab=Description]
Given a binary tree, return the preorder traversal of its nodes' values.
For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?
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