965. 单值二叉树

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def isUnivalTree(self, root: TreeNode) -> bool:
        if root is None:
            return True

        if root.left is not None and root.val != root.left.val:
            return False
        if root.right is not None and root.val != root.right.val:
            return False
        if not self.isUnivalTree(root.left) or not self.isUnivalTree(root.right):
            return False

        return True