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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 root == None:
            return []
        stack = [root]
        res = []
        while len(stack) != 0:
            curNode = stack[-1]
            del stack[-1]
            res.append(curNode.val)
            if curNode.right:
                stack.append(curNode.right)
            if curNode.left:
                stack.append(curNode.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]
        """
        if root == None:
            return []
        self.res = []
        self.dfs(root)
        return self.res
        
    def dfs(self, node):
        if node == None:
            return
        self.res.append(node.val)
        self.dfs(node.left)
        self.dfs(node.right)