965. 单值二叉树

class Solution(object):
    def isUnivalTree(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        if not root:
            return True
        return self.dfs(root)

    def dfs(self, node):
        if not node:
            return True
        if node.left and node.left.val != node.val:
            return False
        if node.right and node.right.val != node.val:
            return False
        return self.dfs(node.left) and self.dfs(node.right)