PU Binary Tree Preorder Traversal

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root: return []
        stack = [root]
        res = []
        while stack:
            node = stack.pop()
            res.append(node.val)
            if node.right: stack.append(node.right)
            if node.left: stack.append(node.left)
        return res

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        def preorder(root, res):
            if not root: return
            res.append(root.val)
            preorder(root.left, res)
            preorder(root.right, res)
        res = []
        preorder(root, res)
        return res