LeetCode - Binary Tree Preorder Traversal

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Problem description is here, or as follows:

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?

• Solution 1: Python
• Solution 2: Python
• Solution 3: Python

The definition of preorder traversal is illustrated as follows:

Comments on Solution 1, 2 and 3 are very similar to those in “Binary Tree Inorder Traversal” problem. See how Solution 2 here is different from the Solution 2 of “Binary Tree Inorder Traversal” and “Binary Tree Postorder Traversal” problems.

Again, Solution 1 and 2 should be memorized.