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花花酱 LeetCode 968. Binary Tree Cameras

Given a binary tree, we install cameras on the nodes of the tree. 

Each camera at a node can monitor its parent, itself, and its immediate children.

Calculate the minimum number of cameras needed to monitor all nodes of the tree.

Example 1:

Input: [0,0,null,0,0]
Output: 1
Explanation: One camera is enough to monitor all nodes if placed as shown.

Example 2:

Input: [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.


Note:

  1. The number of nodes in the given tree will be in the range [1, 1000].
  2. Every node has value 0.

Solution: Greedy + Recursion

Time complexity: O(n)
Space complexity: O(h)

C++