请将下列树的深度优先遍历代码补充完整，横线处应填入（ ）。

Python
复制
1class TreeNode:
2    def __init__(self, x):
3        self.val = x
4        self.left = None
5        self.right = None
Python
复制
1def dfs(root):,[object Object],
2if not root:,[object Object],
3return,[object Object],
4______,[object Object],
5stack.append(root),[object Object],
6while stack:,[object Object],
7node = stack.pop(),[object Object],
8print(node.val, end=" "),[object Object],
9if node.right:,[object Object],
10stack.append(node.right),[object Object],
11if node.left:,[object Object],
12stack.append(node.left),[object Object],
13```,[object Object],