监控二叉树

/**
 * 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:
    int result=0;
    int back(TreeNode* root){
        if(root==NULL) return 1;
        if(root->left==NULL&&root->right==NULL){
            return 0;
        }
        int left=back(root->left);
        int right=back(root->right);
        if(left==3&&right==3){
            result--;
            return 2;
        }
        else if(left==3||right==3){
            return 2;
        }
        else if(!left||!right){
            result++;
            return 2;
        }
        else if(left==2||right==2){
            return 1;
        }
        else{
            result++;
            return 3;
        }
        return 1;
    }
    int minCameraCover(TreeNode* root) {
        back(root);
        return max(result, 1);
    }
};