<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 {
public:
    set<TreeNode*> st;
    int res=0;
    int cnt=0;
    set<int> s;
    void dfs(TreeNode* r, TreeNode* par){
        if(r==NULL) return;
        dfs(r->left,r);
        dfs(r->right,r);
        if(r->left==NULL && r->right==NULL){
            // its a leaf
            if(st.find(par)==st.end()){
                st.insert(par);
                res++;
                s.insert(cnt++);
            }
        }
        else{
            if(st.find(r->left)!=st.end() || st.find(r->right)!=st.end() || st.find(r)!=st.end() || st.find(par)!=st.end()){}
            else{
                st.insert(par);
                res++;
                s.insert(cnt++);
            }
        }
    }

    int minCameraCover(TreeNode* root) {
        dfs(root,root);
        // for(auto it:s) cout<<it<<'\n';
        return res;
    }
};
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