题目：

题解：

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = list()
        if not root:
            return res
        
        p1 = root
        while p1:
            p2 = p1.left
            if p2:
                while p2.right and p2.right != p1:
                    p2 = p2.right
                if not p2.right:
                    res.append(p1.val)
                    p2.right = p1
                    p1 = p1.left
                    continue
                else:
                    p2.right = None
            else:
                res.append(p1.val)
            p1 = p1.right
        
        return res