题目
You are given the root of a binary tree. We install cameras on the tree nodes where each camera at a node can monitor its parent, itself, and its immediate children.
Return the minimum number of cameras needed to monitor all nodes of the tree.
解题思路
贪心维护一个set，存放所有被覆盖的节点，从叶节点向上放，两种情况需要放相机，该节点的左右子树没有相机，或者自己没被覆盖并且是根节点
代码
class Solution:
def minCameraCover(self, root: TreeNode) -> int:
def dfs(node, par = None):
if node:
dfs(node.left, node)
dfs(node.right, node)
if (par is None and node not in covered or
node.left not in covered or node.right not in covered):
self.res += 1
covered.update({node, par, node.left, node.right})
self.res = 0
covered = {None}
dfs(root)
return self.res
