LeetCode Q144: Binary Tree Preorder Traversal (hard)

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?

Round 2 solution:
Very similar to inorder traversal, but save element when first meet.