Binary Tree Preorder Traversal*

Given a binary tree, return the preorder traversal of its nodes' values.

Example

Given:

    1
   / \
  2   3
 / \
4   5

return [1,2,4,5,3].

Solution

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""


class Solution:
    """
    @param root: The root of binary tree.
    @return: Preorder in ArrayList which contains node values.
    """
    def preorderTraversal(self, root):
        # write your code here
        if root == None:
            return []
        stack = [root]
        result = []
        while stack:
            node = stack.pop()
            result.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return result