Preorder Traversal

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?

URL: https://leetcode.com/problems/binary-tree-preorder-traversal/

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param {TreeNode} root
    # @return {integer[]}
    def preorderTraversal(self, root):
        if root == None:
            return []
        else:
            preorderList = []
            stack = []
            stack.append(root)
            while(stack != []):
                node = stack.pop()
                preorderList.append(node.val)
                if node.right:
                    stack.append(node.right)
                if node.left:
                    stack.append(node.left)
            return preorderList