Binary Tree Preorder -JS

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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Tags
Tree, Stack
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Recursive Solution
var preorderTraversal = function (root) {
var ret = [];
preorderTraversalBuilder(root, ret);
return ret;
};

function preorderTraversalBuilder(node, ret) {
if (node === null) {
return;
}

ret.push(node.val);
preorderTraversalBuilder(node.left, ret);
preorderTraversalBuilder(node.right, ret);
}Iterative Solution
Using stack to track all the nodes.
First come last pop. 
- pop() last element
- push right
- push left
We need to guarantee that right node always comes last.

So we push right first, then push left. 
Then pop the last which make left always comes first,
/**
* Definition for a binary tree node.
* function TreeNode(val) {
* this.val = val;
* this.left = this.right = null;
* }
*/
/**
* @param {TreeNode} root
* @return {number[]}
*/
var preorderTraversal = function(root) {
if(root === null) {
return [];
}

var stack = [], ret = [];
stack.push(root);

while(stack.length > 0) {
var cur = stack.pop();
ret.push(cur.val);

if(cur.right) {
stack.push(cur.right);
}

if(cur.left) {
stack.push(cur.left);
}
}

return ret;
};