<p>You are given the <code>root</code> 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.</p>

<p>Return <em>the minimum number of cameras needed to monitor all nodes of the tree</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/29/bst_cameras_01.png" style="width: 138px; height: 163px;" />
<pre>
<strong>Input:</strong> root = [0,0,null,0,0]
<strong>Output:</strong> 1
<strong>Explanation:</strong> One camera is enough to monitor all nodes if placed as shown.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/29/bst_cameras_02.png" style="width: 139px; height: 312px;" />
<pre>
<strong>Input:</strong> root = [0,0,null,0,null,0,null,null,0]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 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.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 1000]</code>.</li>
	<li><code>Node.val == 0</code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
unordered_map<TreeNode*,int> status;
// 0 for needs covering, 1 for camera, 2 for covered but no camera
int count;

bool isNull(TreeNode* node){
    return (node==nullptr || status[node]==1 || status[node]==2 );
}

bool protectParent(TreeNode* node){
    return (node!=nullptr && status[node]==1 );
}

void postOrder(TreeNode* root) {
    if (root==nullptr) return;
    postOrder(root->left);
    postOrder(root->right);

    if (isNull(root->left) && isNull(root->right)) {
        if (protectParent(root->left) || protectParent(root->right))  status[root]= 2;
        else status[root]=0;
    }
    else {
        status[root]=1;
        count++;
    }
}

public:
    int minCameraCover(TreeNode* root) {
        postOrder(root);
        if (status[root]==0){
            status[root]=1;
            count++;
        }
        return count;
    }
};
24900