class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def preorder(root, res):
    if root is None:
        return
    res.append(root.val)
    preorder(root.left, res)
    preorder(root.right, res)
def preorderTraversal(root):
    res = []
    preorder(root, res)
    return res

if __name__ == "__main__":

    root = TreeNode(1)
    root.right = TreeNode(2)
    root.right.left = TreeNode(3)
    result = preorderTraversal(root)
    print("前序遍历结果：", result)