Binary Tree Preorder Traversal LeetCode OJ

# 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):
        list = []
        self.subpreorderTraversal(root,list);
        return list
        
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        
        
    def subpreorderTraversal(self, root, list):
        if(root==None):
            return
        else:
            list.append(root.val)
            self.subpreorderTraversal(root.left, list)
            self.subpreorderTraversal(root.right, list)