参考代码:

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


class Solution:
    def cloneTree(self, root):
        if root is None:
            return None
        clone_root = TreeNode(root.val)
        clone_root.left = self.cloneTree(root.left)
        clone_root.right = self.cloneTree(root.right)