public class Solution
extends Object
968 - Binary Tree Cameras\.
Hard
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_.
**Example 1:**
**Input:** root = [0,0,null,0,0]
**Output:** 1
**Explanation:** One camera is enough to monitor all nodes if placed as shown.
**Example 2:**
**Input:** root = [0,0,null,0,null,0,null,null,0]
**Output:** 2
**Explanation:** At least two cameras are needed to monitor all nodes of the tree. The above image shows one of the valid configurations of camera placement.
**Constraints:**
* The number of nodes in the tree is in the range `[1, 1000]`.
* `Node.val == 0`