0968. Binary Tree Cameras

https://leetcode.com/problems/binary-tree-cameras

Description

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

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