# 二叉树节点类定义
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

# 遍历函数
def traverse(node: TreeNode):
    if not node:
        return
    print(node.val, end=' ')
    traverse(node.left)
    traverse(node.right)