题目：

题解：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def rob(self, root: TreeNode) -> int:
        def _rob(root):
            if not root: return 0, 0  # 偷，不偷

            left = _rob(root.left)
            right = _rob(root.right)
            # 偷当前节点, 则左右子树都不能偷
            v1 = root.val + left[1] + right[1]
            # 不偷当前节点, 则取左右子树中最大的值
            v2 = max(left) + max(right)
            return v1, v2

        return max(_rob(root))